- a symbol for the speed of light. One of the fundamental
principles of physics is that light always travels at the same speed in
a vacuum, exactly 299 792 458 meters per second or about 670 617 300 miles
per hour. Another fundamental principle is that no object can travel faster
than light. At speeds that are large fractions of the speed of light,
the theory of relativity predicts a variety of strange physical effects.
In calculations involving relativity, speeds are customarily expressed as
fractions of the speed of light, such as 0.95 c.
- C 
- the Roman numeral 100, sometimes used as a unit of quantity or as a prefix
meaning 100, as in Cwt (hundredweight) or CCF
(100 cubic feet).
- C 
- a symbol for international standard paper sizes, followed by the size number,
as in C4. The C series of sizes is used primarily for envelopes. A table
of sizes is provided.
- C 
- a unit of relative current for batteries. For a particular battery, a current
of 1C is a current in amperes numerically
equal to the rated capacity of the battery in ampere hours. In other words,
a 1C current will completely charge or discharge the battery in one hour.
- CA, CCA
- abbreviations for "cranking amps" and "cold cranking amps," respectively.
These units are often seen on motor vehicle batteries in the U.S. The amps
involved are ordinary amperes of electric
current. "Cranking amps" measure the current supplied by the battery when
starting the vehicle at a temperature of 32 °F (0 °C), while "cold
cranking amps" measure the current supplied at 0 °F (-17.8 °C).
- a traditional unit of land area in Spanish speaking countries. In Spain
and Peru the caballeria is equal to 60 fanegas,
which is roughly 40 hectares (100 acres). In
Central America it equals 60 manzanas, which
is roughly 45 hectares (110 acres). In Cuba, the caballeria is a smaller unit
equal to 33.162 acres or 13.420 hectares, but in the Dominican Republic it
is a larger unit equal to 1200 tareas or about
75.4 hectares (186.5 acres). In Puerto Rico, the caballeria was equal to 200
cuerdas (see below) which is about 78.6 hectares (194.0 acres).
- caballo de vapor (cv)
- Spanish name for the metric horsepower.
- a unit of distance formerly used at sea. The traditional U.S. mariner's
cable was 120 fathoms long. This is equal
to 720 feet, or 0.1185 nautical mile,
or about 219.4 meters. The British Admiralty, in 1830, defined the cable to
equal exactly 0.1 nautical (Admiralty) mile, which is 608 feet or about 185.3
meters. Some navies are now using a metric cable equal to exactly 200 meters
(about 656.17 ft).
- a traditional unit of volume in Jersey (Channel Islands), used for both
liquid and dry commodities in trade. The cabot equals 10 pots
, which is 17.375 Imperial quarts
or about 19.747 liters. For dry commodities, the cabot is roughly comparable
to 1/2 bushel.
- an old name for a cask, sometimes used as a unit of measure for fish. A
cade of herring, for example, was 720 fish.
- calendar year (cal yr)
- a civil unit of time, equal to 365 days or (in leap years) 366 days. See
year . In archaeology, climatology,
and other sciences studying the earth over the last 40 000 years or so, a
careful distinction must be made between calendar years (cal yr) and radiocarbon
years (14C yr).
- caliber (cal) 
- a unit used to express the bore of a gun. (The bore is the inside diameter
of the gun barrel.) Traditionally, the diameter was stated in inches, so
".22 caliber" referred to a pistol having a bore of 0.22 inches (5.588 mm).
This usage is declining, because bore diameters of many guns are now stated
directly in millimeters. "Caliber" is the American spelling; elsewhere the
unit is often spelled "calibre." The decimal point is usually omitted when
caliber measurements are spoken (as in "38-caliber"). Sometimes it is
omitted when the measurement is written, but this is not a good practice.
- caliber (cal) 
- a measure of the relative length of a gun barrel, defined as the length
divided by the diameter of the bore. Thus a 50-caliber gun on a warship has
a barrel 50 times longer than its bore. Confining the shell within the barrel
for a longer time increases the velocity, so guns with a higher caliber usually
have a longer range.
- the thickness of a sheet of paper or card stock. Traditionally measured
in points  (thousandths of an inch),
caliper is now measured in microns (micrometers). The word "caliper" is sometimes
used in place of the proper unit, as in ".004 caliper" (.004 inch or 4 points)
or "120 caliper" (120 microns).
- call second (Cs)
- a unit of telecommunications traffic equal to one or more calls or other
communications having an aggregate duration of one second. The call
minute (Cmin) and call hour (Ch) are defined similarly.
- Callipic cycle
- a unit of time equal to 76 years or 4 Metonic
cycles, formerly used in astronomy in predicting the phases of the Moon.
After the passage of one Callipic cycle, the phases of the Moon repeat essentially
on the same calendar dates as in the preceding cycle. The cycle is named for
the Greek astronomer Callipus, who discovered it in 330 BCE.
- calorie (cal)
- the CGS unit of heat energy. This calorie (also
called a gram calorie or small calorie)
is the amount of heat required at a pressure of one atmosphere
to raise the temperature of one gram of water by one degree Celsius. Unfortunately,
this varies with the temperature of the water, so it is necessary to specify
which degree Celsius is meant. A traditional choice was the degree from 14.5°C
to 15.5°C; raising the temperature of water through this range requires
4.1858 joules, a quantity called the 15° calorie. Another choice
produces the thermochemical calorie, equal to exactly 4.184 joules.
More common today is the international steam table calorie, or IT
calorie for short, defined by an international conference in 1956 to equal
exactly 4.1868 joules, exactly 1.163 milliwatt
hours, or about 0.003 968 32 British thermal units
(Btu). The name of the unit comes from the Latin calor, heat.
- Calorie (kcal or Cal)
- a common name for the MKS unit of heat energy.
This unit is properly called the kilocalorie; it is also called the
kilogram calorie or large calorie. It is
often (but certainly not always!) distinguished from the small calorie by
capitalizing its name and symbol. The large calorie, or rather kilocalorie,
is the amount of heat required at a pressure of one atmosphere
to raise the temperature of one kilogram of water by one degree Celsius. Since
this is 1000 times as much water as mentioned in the definition of the small
calorie, the kilocalorie equals 1000 small calories, 4.1868 kilojoules,
3.9683 Btu, or 1.163 watt
hours. (These conversions assume the IT calorie is in use; see previous entry.)
These are the "calories" that joggers are trying to get rid of, the ones we
gain by eating. The use of the same term "calorie" for two different-size
units is endlessly confusing, but we seem to be stuck with it.
- caña, canna, canne
- traditional units of distance in Spain, Italy, and southern France, respectively.
The caña varied in size, but it was most often defined as 8 palmos,
which makes it the Mediterranean version of the fathom,
equal to roughly 2 meters (6.5 feet). In Italy a measuring stick is still
called a canna metrica. The unit is sometimes translated "rod" in English,
but "fathom" is the proper choice.
- candela (cd)
- the SI base unit for
measuring the intensity of light. Candela is the Latin word for "candle."
The unit has a long and complicated history. Originally, it represented the
intensity of an actual candle, assumed to be burning whale tallow at a specified
rate in grains per hour. Later this definition was replaced with a definition
in terms of the light produced by the filament of an incandescent light bulb.
Still later a standard was adopted that defined the candela as the intensity
of 1/600 000 square meter of a "black body" (a perfect radiator of energy)
at the temperature of freezing platinum (2042 K) and a pressure of 1 atmosphere.
This definition has also been discarded, and the candela is now defined to
be the luminous intensity of a light source producing single-frequency light
at a frequency of 540 terahertz (THz) with
a power of 1/683 watt per steradian,
or 18.3988 milliwatts over a complete sphere centered at the light source.
The frequency of 540 THz corresponds to a wave length of approximately 555.17
nanometers (nm); normal human eyes are more sensitive to the green light of
this wavelength than to any other. In order to produce 1 candela of single-frequency
light of wavelength l, a lamp would have to radiate 1/(683V(l))
watts per steradian, where V(l) is the relative sensitivity
of the eye at wavelength l. Values of V(l), defined by
the International Commission on Illumination
(CIE), are available online from the Color and
Vision Research Laboratories of the University of California at San Diego
and the University of Tübingen, Germany.
- candle (cd)
- an older name of the candela (see above), or of the candlepower (see below).
- candlepower (cp)
- a unit formerly used for measuring the light-radiating capacity of a lamp
or other light source. One candlepower represents the radiating capacity
of a light with the intensity of one "international candle," or about 0.981
candela as now defined. Since 1948 the candela has been the official SI
unit of light intensity, and the term "candlepower" now means a measurement
of light intensity in candelas, just as "voltage" means a measurement of
electric potential in volts.
- a traditional weight unit of South Asia. The candy was quite variable, generally
within the range 500 to 800 pounds (225 to 365 kilograms). In the international
cotton trade, the candy was generally equal to exactly 7 (British) hundredweight,
which is 784 pounds or 355.62 kilograms.
- can sizes
- Food cans are identified by their nominal dimensions, diameter × height.
(The "nominal" dimensions are somewhat larger than the actual dimensions,
as is the case for lumber and some other products.) In the metric world the
dimensions are 2- or 3-digit numbers representing dimensions in millimeters.
In traditional U.S. nomenclature, the dimensions are stated as 3-digit numbers,
with the first digit representing inches and the remaining two digits representing
16ths of an inch. A common can for fruits and vegetables, for example, is
designated 83 × 116 in metric terminology, or 307 × 409 (3-7/16
× 4-9/16) in traditional terminology. If only one number is mentioned
it is the diameter; thus a "404" can has a nominal diameter of 4-4/16 = 4.25
inches and a "65" can has a nominal diameter of 65 millimeters. Link:
chart of standard
can sizes with metric and traditional designations, from Dantraco Associates.
- an English spelling for the Arab form of the quintal.
In recent years, the cantar has been interpreted as a metric unit equal to
50 kilograms (110.23 pounds); traditional cantars tended to be a few percent
larger than this.
- in rowing, a "canvas" is the distance between the bowman and the bow, or
between the coxswain and the stern. These areas were once covered by canvas.
Winning by a canvas in rowing is analogous to winning by a head in a horse
- Cape foot (cf)
- a traditional unit of distance in South Africa. The Cape foot equals 12.396
English inches, 1.0330 English foot, or 31.4858 centimeters. This unit is
not the traditional Dutch foot, but it is similar in length to the "Rhine
foot" of northern Germany. The Cape foot was widely used for land measurement
and appears on many deeds in South Africa. Europeans often referred to South
Africa as "The Cape," meaning the Cape of Good Hope.
- Cape rood
- a traditional unit of distance in South Africa, equal to 12 Cape feet or
12.396 English feet (3.7783 meters). See also rood.
- carat (ct or c) 
- a unit of mass used for diamonds and other precious stones. Originally spelled
karat, the word comes from the Greek keration, a
carob bean; carob beans were used as standards of weight and length in ancient
Greece in much the same way barleycorns
were used in old England. Traditionally the carat was equal to 4 grains.
The definition of the grain differed from one country to another, but typically
it was about 50 milligrams and thus the carat was about 200 milligrams. In
the U. S. and Britian, the diamond carat was formerly defined by law to be
3.2 troy grains, which is about 207 milligrams. Jewelers everywhere now use
a metric carat defined in 1907 to be exactly 200 milligrams.
- carat (ct or c) 
- in Britain, the spelling "carat" is also used for the unit of gold purity
known in America as the karat.
- carb 
- an informal unit used in the treatment of diabetes, equal to 15 grams of
carbohydrates. This unit is known under various names, including carbo,
carb unit, choice, or exchange. The significance of 15
grams is that in a very rough way that quantity of carbohydrate requires about
1 unit of injected insulin for patients with Type I diabetes (the actual ratio
between carbohydrate and insulin varies considerably from patient to patient
and is usually much lower for patients with Type II diabetes).
- carb 
- an informal unit equal to 1 gram of carbohydrate, commonly used in describing
low-carbohydrate diets such as the Atkins diet. This newer usage of the term
"carb" conflicts with the traditional use by diabetics (previous entry).
- a former French unit for measuring the intensity of light. The unit was
defined as the intensity of a standard Carcel lamp, which burnt colza oil
in a precisely defined way. One carcel equals about 9.74 candelas (see above).
- a traditional unit in Spanish and Portuguese speaking countries. The word
means "load". It was often used as a unit of mass or weight equal to 3 quintals
or as a unit of volume equal to the volume holding 3 quintals of the commodity
- Carnegie unit
- a unit of academic credit used in college admissions decisions in the U.S.
The unit was introduced by the Carnegie Foundation for the Advancement of
Teaching in 1914 to provide colleges with a standard measure of students'
course work in high schools. A Carnegie unit represents the equivalent of
one academic year of study in a subject in a class meeting 4 or 5 times a
week for 40 to 60 minutes per meeting, a minimum of 120 hours of total class
- a traditional unit of land area in Haiti equal to approximately 1.29 hectares
(3.18 acres). The unit originated as the area
of a square 100 pas (Haitian paces) on a side, with the pas being equal to
3.5 pieds (French feet). Foreigners are
not allowed to own more than 1 carreau of urban land or 5 carreaux of rural
land in Haiti.
- a unit of volume, generally informal, equal to the capacity of a small cart.
In Newfoundland, a cart of salt traditionally equalled 6 tubs or 108 Imperial
gallons (490.98 liters).
- a small container. The size of a carton is usually not standardized, but
certain sizes are customary. In the U.S. citrus fruit industry, a standard
carton is equal to 1/2 box or 0.8 bushel
- another name for the hide, an old English
unit of land area. The name comes from a Latin word meaning "plowland."
- a conventional unit of sales for many items, varying with the item and over
time. A case of wine, for example, is traditionally twelve 750-milliliter
bottles. The word comes from the Latin capsa, a chest.
- a unit of angle measure equal to 1/6 turn or 60°, used in mechanical
engineering. A castellated nut is a locknut having a raised rim with a number
of equally spaced slots, usually 6. A cotter pin fits into one of the slots
and into a hole bored in the bolt, holding the nut in place with a precise
degree of torque, or "tightness". To turn the nut one castellation is to turn
it from one slot to the next, that is, by 1/6 turn.
- category (cat)
- the ranking of a hurricane on the Saffir-Simpson
scale, used by the U.S. National Weather Service. A somewhat different scale
of categories is used for tropical cyclones
by the Australian Bureau of Meterology.
- a weight unit of the colonial period in East and Southeast Asia, originating
as the kati in Malaya. The catty varied a little from market to market.
Typically it was equal to about 4/3 pound avoirdupois
(604.79 grams), and it is still equal to that weight in Malaysia. In Thailand,
the catty is used now as a metric unit equal to exactly 600 grams (1.3228
pounds). In China, the catty was identified with the jin,
a traditional Chinese unit.
- cawny or cawney
- a traditional unit of land area in southern India, equal to about 4/3 acre
(0.54 hectare). The name is an English tranliteration
of a Tamil word for the unit.
- abbreviation for "cubic," seen in combinations such as cbm (cubic
meter) or cbft (cubic foot). The proper symbol for cubic meters is
m3, not cbm.
- cc 
- an alternate symbol for the cubic centimeter (cm3). This symbol
is obsolete and should not be used; cm3 should be used in its place.
The cubic centimeter is the same volume as the milliliter (mL or ml).
- cc or cc 
- a symbol for the centesimal second (see below).
- an abbreviation for 100 cubic feet. Local water and sewer utilities often
sell water in CCF units; for this purpose one CCF equals about 748.05 gallons
(U.S.) or about 2831.7 liters. Utilities sometimes sell natural gas in CCF
units; for this purpose the CCF is really a unit of energy roughly equivalent
to the therm (1 CCF of natural gas provides
about 1.034 therm).
- an incorrect symbol for the cubic centimeter (cm3).
- a unit of telecommunications traffic equal to 100 call-seconds. One CCs
could represent a single call 100 seconds long, or 10 calls each 10 seconds
long, etc. The symbol stands for "centum call second."
- CE 
- abbreviation for "common era." This abbreviation is a non-religious designation
used in place of the traditional AD for years of the common or Christian era.
Years of the common era are supposed to be counted from the birth of Jesus
of Nazareth, founder of the Christian religion. However, the year-numbering
system was not established until more than 500 years later. It is based on
calculations of the priest and scholar Dionysius Exiguus placing Jesus's birth
in the Roman year 753 AUC. Dionysius knew he
had incomplete information, and there is evidence that he picked this particular
date to simplify the calculation of the date of Easter. According to the Biblical
account, Jesus was born several years before the death of Herod the Great,
who died, we now know, in 750 AUC (4 BC). Thus the calculation of the common
era is off by 6 or 7 years at least. In the conventional use of the common
era system, there is no year 0 and the year prior to 1 CE is designated 1
BCE (or 1 BC). In astronomy, however, it simplifies calculations to define
the year 0 CE = 1 BC and to apply negative numbers to earlier years. Thus
Herod died in -3 CE, and, in general, -n CE is the year more commonly
called n + 1 BC.
- CE 
- French abbreviation for colonne d'eau, water column, seen in pressure
measurements. See centimeter of water (below) or millimeter
- see degree Celsius. The word "degree" is
often omitted in informal statements of temperature, as in "we expect a high
of about 23 Celsius today."
- Celsius heat unit (Chu)
- a unit of heat energy equal to the energy required to raise the temperature
of one pound of water by 1°C at standard atmospheric pressure. 1 Chu
is equal to exactly 1.8 Btu, approximately 453.59
IT calories (see above), or 1.8991 kilojoules.
The unit is also called the centigrade heat unit.
- cent 
- an old English unit of quantity, usually equal to 100 but sometimes 120
(the great or long hundred) or some other figure of similar
size. The Latin number 100, centum, is also used in English works for
- cent 
- an informal name for 1/100 of almost any unit, for example, a centiliter
(0.01 liter). Phrases such as "15 cents of an inch" were formerly common in
- cent 
- a unit used in music when it is necessary to specify the ratio in frequency
between two tones with great precision. There are 100 cents in a semitone,
or 1200 in an octave. If two notes differ by 1 cent, the ratio between their
frequencies is 21/1200 or approximately 1.000 5778. One cent
equals 1/1.2 = 0.83333 millioctave or
about 0.2509 savart.
- cent 
- a unit used in nuclear engineering to describe the "reactivity" of a nuclear
reactor, equal to 0.01 dollar. For a discussion
of reactivity, see inhour.
- cent 
- a common unit of land area in southern India, equal to 0.01 acre,
exactly 435.6 square feet, or about 40.47 square meters.
- cental (cH or ctl)
- an alternate name in Britain for the U.S. hundredweight,
which is equal to exactly 100 pounds (the British hundredweight is 112 pounds).
Introduced by British merchants around 1850, the name was apparently coined
after the model of the quintal. The cental
has sometimes been confused with the centner (see below).
- centesimal minute (c), centesimal second
- units of angle measure sometimes used in surveying. In the centesimal system,
the right angle is divided into 100 grads or
gons. Each gon is divided into 100 centesimal minutes (or centigons) and each
centesimal minute into 100 centesimal seconds. Thus the centesimal minute
equals 0.009° or 0.54 arcminute, and the centesimal second equals 0.324
- centi- (c-)
- a metric prefix meaning one hundredth, or 0.01. The prefix comes from the
Latin word centum for one hundred.
- centiare (ca)
- a metric unit of area. The centiare equals 0.01 are,
which is exactly 1 square meter (about 10.7639 square feet).
- centibar (cbar or cb)
- a metric unit of pressure identical with the kilopascal
(kPa). One centibar equals 0.01 bar, 7.5006 torr,
or 0.1450 pounds per square inch (lbf/in2 or psi). The centibar
is traditionally used in agriculture as a unit of soil water tension (the
water pressure on the roots of plants) as measured by devices called tensiometers.
- centigon (cgon)
- a unit of angle measure equal to 0.01 gon,
0.01 grad, or 0.0001 right angle; this is equivalent to 0.009°, 0.54
arcminute, or exactly 32.4 arcseconds. The centigon is useful in navigation
(potentially, at least) because 1 centigon of latitude represents approximately
1 kilometer on the earth's surface, in the same way that 1 nautical mile
1 minute of latitude in traditional navigation.
- centigrade 
- a temperature scale; see degree centigrade.
- centigrade 
- a French unit of angle measurement equal to 0.01 grad, 0.009°, 0.54', or 32.4".
- centigram (cg)
- a metric unit of mass equal to 10 milligrams or about 0.154 grain.
- centigray (cGy)
- a unit of radiation dose equal to 0.01 gray
or 1 rad. Dose of radiation used in cancer treatments,
formerly stated in rads, are generally stated now in centigrays.
- an informal unit of pressure equal to 1 centimeter of mercury. This is equivalent
to 10 millimeters of mercury, approximately 0.3937 inHg,
0.1933 lb/in2, 13.33 millibars,
or 1333 pascals. The word is pronounced "sentig".
- centiliter (cl or cL)
- a common metric unit of volume. One centiliter equals 10 cubic centimeters;
this is about 0.610 24 cubic inch, 0.3318 U.S.fluid
ounce or 0.3519 British fluid ounce. In the kitchen, a centiliter is roughly
equal to 2 U.S. teaspoons (or 0.704 British
- centimeter (cm) 
- the basic unit of distance in the CGS version
of the metric system, equal to 0.01 meter. One centimeter is about 0.393 700
- centimeter (cm) 
- an obsolete name for the statfarad (approximately
1.11 picofarad), the CGS electrostatic unit
- centimeter (cm) 
- an obsolete name for the abhenry, the CGS electromagnetic
unit of inductance. The abhenry is the same as the nanohenry.
- centimeter (cm) 
- one of several traditional units of pressure, including the centimeter
of mercury and the centimeter of water (next two entries).
- centimeter of mercury (cmHg, cm Hg)
- a traditional unit of pressure equal to 10 mmHg, 1.333 22 kilopascal,
or about 0.193 pounds per square inch.
- centimeter of water (cmH2O, cm WC, cm
CE, cm WS)
- a unit of pressure equal to the pressure exerted at the Earth's surface
by a water column (WC) 1 centimeter high. This is about 98.067 pascals,
0.980 67 millibars, 0.3937 inch
of water, or 2.04 pounds per square foot. The unit is used in respiratory
medicine and elsewhere to measure air pressures. The French symbol is cm CE
(colonne d'eau), and the German symbol is cm WS (Wassersäule).
- a word sometimes used to mean 100 million (108).
This usage applies the traditional Latin prefix cent-, meaning 100, but it
conflicts with the metric prefix centi-, which means 1/100. In metric language,
the number 100 million could be called a hectomillion or (at least in the
U.S.) a decibillion.
- centimorgan (cM)
- a unit of genetic separation used in genetics and biotechnology. If two
locations on a chromosome have a 1% probability of being separated during
recombination in a single generation, then the distance between those locations
is one centimorgan. In humans, the centimorgan is approximately equal to one
million base pairs. The unit honors the pioneering American geneticist Thomas
Hunt Morgan (1866-1945), who received the 1933 Nobel Prize in Medicine for
his discoveries concerning the role played by the chromosome in heredity.
- centinewton (cN)
- a metric unit of force equal to 0.01 newton.
This unit has some popularity in engineering as a substitute for the gram
of force (gf), since it equals about 1.019 72 gf (about 0.0360 ounces of force
in the English system). In the textile industry, the breaking strength of
fibers is commonly expressed in centinewtons per tex.
- centipoise (cP, cPs, or cPo)
- a common metric unit of dynamic viscosity equal to 0.01 poise
or 1 millipascal second (mPa·s). The dynamic viscosity of water at 20
°C (68 °F) is about 1 centipoise. The correct symbol for the unit
is cP, but cPs, cPo, and even cps are sometimes used.
- a unit of angle measure equal to 0.01 radian
or about 0.572958° (34' 22.65").
- centisecond (cs or csec)
- a unit of time equal to 0.01 second or 10 milliseconds. Centiseconds are
frequently used in the study of human speech to measure precisely the length
- centistokes (cSt)
- a common metric unit of kinematic viscosity equal to 0.01 stokes
or 1 mm2/s. The viscosity of lubricating oils and many other liquids
is frequently stated in centistokes. Although the centistokes is not an SI
unit it is likely to remain in use, since it provides a convenient and traditional
name for an SI-appropriate quantity (1 mm2/s). The older symbol
cS should not be used for this unit, since S is now the symbol for
- centner 
- the English name for a German weight or mass unit, the zentner,
equal to 50 kilograms or about 110.231 pounds.
The name centner should not be used for the cental (see above).
- centner 
- a Russian weight or mass unit equal to 100 kilograms (approximately 220.4623
pounds). This centner, also used in Ukraine
and the other former Soviet republics, is equal to the decitonne and to the
metric quintal; it is twice the size of the
centner  used in western Europe.
- an informal name for the centiradian (see above).
- the Latin number 100, sometimes used in works in English.
- century 
- a unit of quantity equal to 100. In ancient Rome, a "century" was originally
a company of about 100 soldiers led by an officer called a centurion.
- century 
- a traditional unit of time equal to 100 years. In naming centuries, historians
recall that there was no year 0 in the conventional year numbering system.
Thus the First Century included the years 1-100 and the Twentieth Century
included the years 1901-2000. (As an example in the other direction, the Fifth
Century BC included the years 500-401 BC.) With this convention, 2001 is the
first year of the Twenty-first Century.
- cetane number
- a measure of the ability of diesel fuel to reduce engine knocking. The cetane
number plays the same role in diesel engine technology that the octane
number plays in conventional automobile engine technology. It is the percentage
by volume of cetane which must be added to methylnaphthalene to give the mixture
the same resistance to knocking as the diesel fuel sample being tested. Cetane
is the name of a hydrocarbon compound whose molecules contain 16 carbon atoms
and 34 hydrogen atoms, the 16 carbons being arranged in a long chain. Adding
one oxygen atom to cetane produces cetyl alcohol, a waxy compound found in
whale oil. The words "cetyl" and "cetane" are both derived from the Latin
word cetus for a whale.
- a common symbol for the cubic foot.
- a symbol for cubic foot of gas equivalent, used in the natural gas industry.
This is really an energy unit; 1 cfe is equal to about 1034 British
thermal units (Btu), 0.01034 therm, or
1.091 megajoule (MJ). Multiples of this unit
are formed using non-metric prefixes: Mcfe for 1000 (not 1 million)
cfe, MMcfe for 1 million cfe, and Bcfe for 1 billion cfe.
- cfh, cfm, cfs
- traditional abbreviations for cubic feet per hour, cubic feet per minute,
and cubic feet per second, respectively. 1 cfm = 28.3169 liters per minute
(L/min) and 1 cfs = 28.3169 liters per second (L/s). Symbols with a slash,
such as Cf/h, are also seen.
- abbreviation for "colony forming units," a count of the number of active
bacterial cells in preparations of Lactobacillus acidophilus and other
"friendly" organisms of the digestive system. Counts as high as one billion
CFU per gram are not uncommon.
- chain (ch)
- a unit of distance used or formerly used by surveyors. Although the unit
is not often used today, measured distance along a road or railroad is commonly
called chainage regardless of the units used. The traditional British
surveyor's chain, also called Gunter's chain because it was introduced
by the English mathematician Edmund Gunter (1581-1626) in 1620, is 4 rods
 long: that's equal to exactly 1/80 mile, 1/10 furlong,
22 yards, or 66 feet (20.1168 meters). The traditional length of a cricket
pitch is 1 chain. Gunter's chain has the useful property that an acre
is exactly 10 square chains. The chain was divided into 100 links.
American surveyors sometimes used a longer chain of 100 feet, known as the
engineer's chain or Ramsden's chain. (However, Gunter's chain
is also used in the U.S.; in fact, it is an important unit in the Public Lands
Survey System.) In Texas, the vara chain of 2 varas (55.556 ft) was
used in surveying Spanish land grants. In the metric world, surveyors often
use a chain of 20 meters (65.617 ft). See also shackle
and shot  for anchor chain lengths.
- chain number
- a size designation for roller chains, such as the drive chains of bicycles
or motorcycles. These chains are traditionally designated by a three-digit
number. The first digit specifies the pitch, the distance between pins, in
eighths of an inch; the second and third digits
specify the width of the rollers in 80ths of an inch. Thus a 425 chain has
a pitch of 4/8 (= 0.5) inch and a roller width of 25/80 (= 0.3125) inch.
- chalder or chaldron (chd)
- a traditional British unit of volume or weight used for dry commodities
such as coal or lime. As a volume measure, the chaldron is equal to 36 bushels,
or 288 British Imperial gallons; this is equivalent
to 46.237 cubic feet or 1.3091 cubic meters. As a measure for coal, the chalder
equals 1/8 keel or 53 hundredweight
(5936 pounds or 2692.52 kilograms). The words "chalder," "chaldron," and "cauldron"
are English spellings of the same old French word, which originally meant
a large kettle.
- champagne quart
- see quart .
- character (char)
- a unit of information used in computer science and telecommunications.
The number of bits required to specify a character has varied as more robust
coding systems have been developed. By the 1970s, one character was typically
equal to 8 bits or
Current coding systems such as UTF-8 use up to 4 bytes to designate
all the characters used in all the world's languages and other symbolic
- a traditional Russian unit of volume containing about 123.0 milliliters,
4.159 U.S. fluid ounces or 4.329 Imperial fluid ounces. There are 6.25 charki
in a boutylka (bottle) and 10 in a schtoff.
The word charka means a cup or glass.
- Charrière, Charrière gauge (Ch)
- a unit of distance used for measuring the diameters of small tubes such
as catheters, fiber optic bundles, etc. The gauge number is the diameter of
the tube in units of 1/3 millimeter. In English-speaking countries, the scale
is usually called the French gauge and the unit is simply called "French."
Charrière was a nineteenth century French instrument manufacturer.
- cheval vapeur (cv or ch)
- French name for the metric horsepower.
In Canadian French, however, the term is used to mean the English horsepower.
- chǐ, chi or ch'ih
- a traditional unit of distance in China, sometimes called the Chinese foot. In modern China, the chǐ equals
10 cùn, exactly 33-1/3 centimeters, or 13.123 356 inches.
There are 1500 chǐ in a lǐ.
- chiliad 
- a unit of quantity equal to 1000. The word comes from the Greek numeral
1000, chilioi, which is also the origin of the metric prefix kilo-.
Pronounced "killiad," the chiliad was once fairly common in learned writing,
but it has nearly disppeared from use today.
- chiliad 
- another name for a millennium (1000 years).
- an informal Italian name for a kilogram.
- Italian spelling of the metric prefix kilo- (1000).
- one of several spellings in English for the jin,
a traditional Chinese weight unit.
- in New Zealand, another name for a punnet.
- a name used in Thailand and Laos for the viss.
The word is sometimes spelled joi in English.
- a traditional French unit of volume. The unit varied regionally, but by
the 18th century it was more or less standardized as 23.475 cubic pouces
(465.7 milliliters). The chopine is obsolete in France today, but the word
survives (especially in Canada) as a French name for the English pint
- a traditional Scottish unit of volume equal to 2 mutchkins
or 1/2 Scots pint. The choppin is equivalent
to about 52.1 cubic inches, 1.80 U.S. liquid pints, 1.50 British Imperial
pints, or 854 milliliters.
- CI or ci
- a traditional symbol for the cubic inch.
- cicero (cc)
- a unit of distance used by typesetters and printers in continental Europe,
equal to 12 Didot points. This is approximately
0.1780 inch or 4.52 millimeters. The cicero corresponds to the British and
American pica. Presumably, this unit got its
name because type of this size was used in printing the works of classical
authors such as the Roman statesman and orator Marcus Tullius Cicero (106-43
- a symbol for cubic inch of displacement, formerly used in the U.S. in stating
the engine displacements of motor vehicles; these measurements are now made
- an old English word for the number 5, pronounced "sink" and derived from
the French number 5, cinq. In English history, the original Cinque
Ports were Sandwich, Dover, Hythe, Romney and Hastings. The word survives
today as the name for a 5-spot showing in dice, or for a 5-card in card games.
- circle (cir)
- the traditional unit of angle measure, divided into 2 pi
radians or 360 degrees.
- circular inch, circular mil (cmil)
- informal units of area. A circular inch is the area of a circle one inch
 in diameter, and a circular mil is the area of a circle one mil
 in diameter. A circle of diameter d has an area of pi·d2/4,
so the circular inch is equal to approximately 0.785 398 square inches or
5.067 07 square centimeters, and the circular mil is equal to approximately
0.785 398 square mils or 506.707 square micrometers.
- city block
- see block.
- civil year
- a year as measured by the conventional (Gregorian) calendar, equal to 365
days in most years but 366 days in a leap year. See year
. This is the same unit as the calendar year (see above). Both
names are often used to specify years beginning with January 1, as opposed
to a fiscal year beginning on some other date.
- Clark degree
- see degree .
- a unit of entropy. Entropy is a measure of the extent to which heat or energy
in a physical system is not available for performing work. It is computed
in units of energy per kelvin. One clausius
is equal to 1 kilocalorie per kelvin (kcal/K) or 4.1868 kilojoules
per kelvin (kJ/K). The unit honors the German physicist Rudolf Clausius (1822-1888),
who introduced and named the concept of entropy in 1850.
- U.S. military slang for the kilometer
(about 0.621 mile). Also spelled klick
or klik. This unit became popular during the Vietnam War, but it was
invented by U.S. troops in Germany during the 1950s. Occasionally it was used
as a non-metric unit equal to 1000 yards (0.9144
- city block
- see block.
- a unit of thermal insulation used for clothing. One clo is intended to represent
the insulation required to keep a resting person warm in an indoor room at
70 °F (21.1°C). The rate of a person's heat loss is measured in
watts per square meter of skin area per kelvin of temperature difference across
the clothing; the value of insulation is measured by the reciprocal of this
rate, in square meter kelvins per watt (m2K/W). One clo is equal
to 0.155 m2K/W or 1.550 togs.
- clothyard, clothier's yard
- an alternate name for the ell. The English
ell is 45 inches (1.143 meters), but the "clothyard arrows" used with longbows
in late medieval times were closer in length to the 37-inch Scottish ell.
- an old English unit of weight. A clove is usually considered equal to 1/2
stone or 1/16 hundredweight;
that's 7 pounds (3.175 kilograms) by the modern
definition of the stone, but in the past the clove varied from 6.25 to 8 pounds.
- a unit of power used to express the performance or leakage of vacuum pumps.
One clusec represents a flow of 10 milliliters per second at a pressure of
one micrometer (or micron) of mercury. This
is equivalent to 0.01 lusec or 1.333 microwatts.
The name of the unit is an acronym for "centi-lusec."
- a unit of relative electric current used especially in connection with nickel
metal hydride (NiMH) storage batteries. The symbol designates the current
flow per hour, into or out of the battery, as a fraction of the battery's
rated capacity. In other words, a current of 0.1 CmA would completely charge
or discharge the battery in 10 hours. Put another way, if the rated capacity
of the battery is 2 ampere hours or 2000 milliampere hours, then 0.1 CmA is
a current flow of 200 milliamperes.
- symbol for the circular mil (see above). Note that this is not a
- coffee measure
- a flat-bottomed scoop or spoon used to measure coffee in U.S. homes. The
coffee measure holds 2 U.S. tablespoons
(about 29.57 milliliters).
- a unit of volume formerly used in U.S. food recipes. A coffeespoon is 1/2
teaspoon, 1/12 fluid
ounce, or about 2.5 milliliters.
- an ancient Persian unit of liquid volume, equal to 1/8 artaba
or (in recent centuries) about 8.25 liters.
- color rendering index (CRI)
- a scale used in engineering to measure the ability of an artificial lighting
system to show the "true" colors of objects, that is, the colors those object
display in natural daylight out of doors. The scale is from 0 to 100, with
higher numbers representing a higher degree of fidelity of color. The test
procedure, developed by the Illuminating Engineering Society of North America
(IESNA), involves comparison of eight test colors under both natural lighting
and the artificial source being tested.
- color temperature (CCT)
- a measure of the overall "color" of a light source. The measurement is
obtained by comparing the spectrum, or mix of colors (wavelengths of light),
produced by the light source to the spectrum of a "black body," a theoretical
object that absorbs all radiation falling on it. (A black body is also a
perfectly efficient radiator of energy, its spectrum depending only on its
temperature.). The measurement is expressed in kelvins (K).
Lower temperatures indicate more red and yellow light, higher temperatures
more blue. Incandescent light bulbs are "cool" with a color temperature
of about 2800 K, while daylight at noon is much "hotter" (bluer) at about
6000 K. Fluorescent lighting can be produced with a broad range of color
temperatures including everything between these two values. "CCT" stands
- color units
- several systems have been devised to measure colors. For most of us not
directly concerned with dyes, paints, or inks, the subject was academic until
recently, but now computers require precise methods for describing the colors
to be displayed or printed. These methods typically use three variables, reflecting
the fact that the human eye has three types of color sensor. Computer monitors
use the RGB system, which specifies colors with three variables measuring
the intensity of the three primary colors red, blue, and green in the color.
Frequently each variable is specified by one byte
and therefore takes values in the range 0 to 255. If all three are 0, the
resulting color is black; if all three are 255 the resulting color is white.
The RGB settings for the Carolina blue background of this page are R=153,
G=204, and B=255. Since it is difficult to estimate the relative amounts of
red, green, and blue needed to create a particular color, many graphics design
programs use the HSV color system, which describes colors using three variables
called hue, saturation, and value. Once again, all three
variables are assigned values from 0 to 255. Hue, which is what we call "color"
in ordinary language, is described on a circular scale. Hue values begin with
red at 0 and run through yellow, green, blue, and purple before returning
to red at 255. Saturation is the purity of the color, the extent to which
it is not watered down with gray. The pure color has saturation 255. As saturation
is reduced, the color becomes grayer, until at saturation 0 the color is replaced
by a neutral gray of the same intensity as the original color. The value (or
intensity) of the color is its brightness. The pure or most natural form of
the color has value in the middle of the scale, at 127. As value is increased
the color becomes brighter. In the opposite direction, the color becomes less
bright, becoming black at value 0. This page's background has hue 140, saturation
240, and value 192 in the HSV system. See also: Lovibond
color units (used for beer and honey).
- colpa, colp, or collop
- a traditional Irish unit. The colpa was originally a unit of livestock equal
to one cow or horse or to 6 sheep. Later it was used as a unit of pastureland
equal to the pasturage supporting one colpa of livestock. This varied according
to the quality of the land, but it was roughly equal to the Irish acre (0.6555
hectare). "Collop" is an English version
of the Irish word "colpa."
- column inch (col in)
- a unit of relative area used in journalism. A column inch is an area one
column wide and one inch deep. The width of a column varies; a standard size
in the U.S. is 2-1/16 inch. At this width a column inch is 2.0625 square inches
or about 13.31 square centimeters.
- commercial acre
- a unit of area used in U.S. real estate, equal to exactly 36 000 square
feet or about 0.826 45 ordinary acre (0.334
45 hectare). The unit was invented by commercial
realtors to express the approximate portion of an acre of subdivided land
that remains to be sold in the retail market after portions are set aside
(dedicated) for necessary streets and other utilities. Buyer beware! It is
legal to sell land by the commercial acre in many U.S. states, although most
consumers are not aware of the smaller size of the unit.
- common year
- a year of 365 days, as opposed to a leap year of 366 days.
- cooling degree day (CDD)
- see degree day.
- cone, cone number
- a measure of temperature used by potters. Pyrometric cones are cone-shaped
objects designed to soften and bend after absorbing a specific amount of heat.
Potters place these cones in the kiln and observe them through peepholes;
when the cone bends all the way over the proper amount of heat has been delivered
to the pottery being fired. Although cones bend within narrow temperature
ranges there is not a simple relationship between cone number and temperature.
Technical tables are posted
on the Internet by the Orton Ceramic Foundation.
- a metric unit of area used in Vietnam. One cong equals 1000 square meters,
which is 0.1 hectare, 0.24177 acres,
or 1196.00 square yards.
- a historic unit of liquid volume. The Roman congius was equal to about 3.2
liters (3.4 U.S. quarts or 2.8 British Imperial
quarts); it was divided into 6 sextarii (sixths) which corresponded closely
to modern pints. In the nineteenth century,
the congius was used in British medicine and pharmacology as a name for the
British Imperial gallon (4.546 09 liters).
- coomb or coom
- a traditional British unit of volume used mostly for dry commodities. A
coomb is 4 Imperial bushels; this is equivalent
to 5.1374 cubic feet or about 145.48 liters.
- an abbreviation for coefficient of performance, a measure of the
efficiency of heat pumps, air conditioners, refrigerators, and freezers.
The COP is the ratio of the useful energy output of the system (the amount
of heat energy added to or removed from the building) divided by the electric
energy input when the unit is operating in a steady-state test condition.
Typical values are in the range 2-4. (The energy "output" exceeds
the input, because the system only uses the input energy to move heat energy,
not to create it.) Heat pumps have a higher COP for heating than for cooling,
because the compressor's input energy produces heat energy that can be used
in heating but must be dissipated outdoors for cooling. For air conditioners,
the COP equals the energy efficiency ratio (EER)
divided by 3.412.
- cord (cd) 
- a traditional unit of volume used to measure stacked firewood. Like most
traditional units of trade, the cord has varied somewhat according to local
custom. In the United States, the cord is defined legally as the volume of
a stack of firewood 4 feet wide, 8 feet long, and 4 feet high. (In Maryland,
the law specifies that the wood be stacked "tight enough that a chipmunk cannot
run through it." Presumably it is up to the buyer to provide the chipmunk.)
One cord is a volume of 128 cubic feet, about 3.6246 cubic meters, or 3.6246
steres. The name apparently comes from an old
method of measuring a stack of firewood using a cord or string.
- cord (cd) 
- in the U.S. timber industry, the cord is also used as a unit of weight for
pulpwood. The weight varies with tree species, ranging from about 5200 pounds
(2.36 metric tons) for pine to about 5800 pounds (2.63 metric tons) for hardwood.
- cord foot (cd ft)
- a traditional unit of volume used to measure stacked firewood. A cord foot
is the volume of a stack of firewood 4 feet wide, 1 foot long, and 4 feet
high. Thus the cord foot is 1/8 cord, or 16 cubic feet, or about 0.4531 cubic
- cordel 
- a traditional unit of distance in Spain and Latin America. More specifically,
the cordel is a rope used in land measurement. In Mexico and the southwestern
U.S., the cordel measured 50 varas or about
42.33 meters (138.9 feet), using the Texas standard of 33 1/3 inches for the
vara. In Cuba, however, the cordel was only 24 varas or about 20.35 meters
(66.8 feet). Longer cordels were used in some parts of South America.
- cordel 
- a unit of area equal to one square cordel: about 1792 square meters (2143
square yards or 0.433 acre) in Mexico and the
southwestern U.S. or about 414.2 square meters (495.4 square yards).
- coulomb (C)
- the SI unit of electric charge. One coulomb is the
amount of charge accumulated in one second by a current of one ampere.
Electricity is actually a flow of charged particles, such as electrons, protons,
or ions. The charge on one of these particles is a whole-number multiple of
the charge e on a single electron, and one
coulomb represents a charge of approximately 6.241 506 x 1018 e.
The coulomb is named for a French physicist, Charles-Augustin de Coulomb (1736-1806),
who was the first to measure accurately the forces exerted between electric
- count (ct) 
- a unit of quantity equal to 1. This unit is used in commerce to specify
that the quantity stated represents a reliable count. For example, a carton
marked "oranges 24 ct" contains exactly 24 oranges.
- count (ct) 
- a traditional unit measuring the texture of a fabric, equal to the number
of threads per inch. A 100 count fabric has
39.37 threads per centimeter.
- count (ct) 
- an informal unit of volume in bartending, equal to 0.5 fluid ounce (14.8
milliliters). Bartenders usually fit bottles with pourers designed to restrict
the flow to 0.5 fluid ounce per second. They can then measure a quantity of
liquid by counting, "one thousand one, one thousand two, ..." while pouring.
This is much faster than using a measuring glass, and just about as accurate.
- count (ct) 
- a measure of size used in the U.S. for shrimp and similar items described
by the number of items per pound. Thus "50 count" shrimp weigh an average
of 1/50 pound each.
- English, like most languages, has a procedure for stating the precise relationship
between persons of common descent; a typical designation is "second cousins,
once removed." First cousins are persons sharing a common grandparent; second
cousins are persons sharing a common great-grandparent, and, generally, for
n > 1, n-th cousins are persons sharing a common (n -
1)-times-great-grandparent. This means n-th cousins have n +
1 generations in each of their descents from the common ancestor. The "removed"
phrase is used when the number of generations in the descent from the common
ancestor is not the same for both cousins: "r times removed" means
the difference in the number of generations is r. Thus, for n-th
cousins r times removed, the common ancestor is an (n - 1)-times-great-grandparent
of one cousin and an (n + r - 1)-times-great-grandparent of the other
cousin. (In the case of first cousins r times removed, the grandparent
of one cousin is also an r-times-great-grandparent of the other cousin.)
For n-th cousins r times removed, there are n + 1 generations
in the descent from a common ancestor for one cousin, and n + r + 1
generations for the other. The number of degrees
of consanguinity between n-th cousins r times removed is
2n + r + 2.
- covado, covido
- Portuguese and Arabic names, respectively, for the cubit (see below). The
Portuguese covado is equal to 3 palmos (66 centimeters, or 20.12 inches),
while the Arabic covido is about 48 centimeters or 19 inches.
- a traditional Welsh unit of area, standardized in the British system to
be exactly 2/3 acre (about 0.2698 hectare).
The word is an Anglicized version of the Welsh name cyfair for
- a common abbreviation for characters per inch, used in printing. The unit
is also called pitch.
- a traditional abbreviation for cycles per second; also an incorrect symbol
for the centipoise (see above).
- a traditional unit of volume formerly used by fishermen. The cran, originally
intended to represent the volume of fish held by a barrel, was standardized
at 37.5 Imperial gallons, or about 170.46
liters. This is somewhat larger than the usual Imperial barrel.
In modern times, the cran is also used somewhat informally to mean a volume
of fish weighing about a hundredweight (112
pounds or 50.8 kilograms). The name of the unit is an old Gaelic word meaning
a quantity or measure of something.
- an old English unit of volume for grains. Never standardized, the crannock
was roughly 10 bushels (350 liters).
- a unit of mass sometimes used in the physics and chemistry of gases. The
crith is equal to the mass of a liter of hydrogen at standard temperature
(0.01°C) and pressure (1 atmosphere);
this is about 89.885 milligrams. The name comes from an ancient Greek word
for a barleycorn.
- a traditional unit of quantity in India, equal to 107 or 10 million.
Large numbers are usually described in India using the crore and the lakh
(105); for example, the number 25 600 000 is called 2 crore 56
lakh and written "2,56,00,000".
- a unit of relative time in music equal to 1/4 whole note or 1/8 breve.
The word, pronounced crotch-it, comes from the old Norse word krok
for a hook; in this context it refers to the traditional hooked symbol for
a quarter note.
- a unit of information in computer science, equal to 2 bits.
The unit is thought to have originated at IBM in the early 1980s. There are
2 crumbs in a nibble.
- abbreviation for "cubic," as in cubic feet or cubic meters. This symbol
is forbidden by the SI, but it remains in common use
in ordinary English text. The proper symbol for cubic feet is ft3,
not cu. ft.
- cuadra 
- a traditional Latin American unit of distance. The cuadra is generally equal
to 100 varas (about 84 meters or 275 feet) in
Central America and northern South America. In Argentina and Chile, the cuadra
is equal to 150 varas (roughly 130 meters or 410 feet).
- cuadra 
- a traditional Latin American unit of area equal to one square cuadra .
Except in Argentina and Chile, this is 10 000 square varas, generally in the
range 1.75-1.85 acres (0.71-0.75 hectare). In Argentina and Chile, the cuadra
was 22 500 square varas (4.18 acres or 1.69 hectare).
- a traditional Spanish unit of volume comparable to the liter or the English
quart. The cuartillo equals 4 octavillos
or 1/4 almude and contains 1.156 25 liters,
which is about 1.222 U.S. liquid quart or 1.017 British Imperial quart.
- cubic centimeter (cm3 or cc)
- the CGS unit of volume, equal to 10-6
cubic meter, 1 milliliter, or about 0.061 023 7 cubic inch.
- cubic foot (ft3, cu ft, or cf)
- a traditional unit of volume in English speaking countries. One cubic foot
equals 1728 in3, 1/27 yd3, 0.028 316 85 m3,
or 28.316 85 liters. The cubic foot also holds about 7.4805 U.S. liquid gallons
or about 6.2288 British Imperial gallons.
- cubic inch (in3, cu in, or CI)
- a traditional unit of volume in English speaking countries. One cubic inch
equals 1/1728 = 5.787 037 x 10-4 ft3, 16.3871 cm3,
16.3871 milliliters, 0.5541 U.S. fluid ounce, or 0.5767 British Imperial fluid
- cubic meter (m3)
- the SI unit of volume, equal to 106 cm3,
1000 liters, 35.3147 ft3, or 1.307 95 yd3. A cubic meter
holds about 264.17 U.S. liquid gallons or 219.99 British Imperial gallons.
- cubic ton
- a unit of volume, used in various ways but most commonly equal to 40 cubic feet (1.13267 cubic meters). This makes it the same as the freight ton (see ton ).
- cubic yard (yd3)
- a traditional unit of volume in English speaking countries. One cubic yard
equals 27 ft3, 46 656 in3, 0.764 555 m3,
or 764.555 liters. A cubic yard holds about 201.97 U.S. liquid gallons or
about 168.20 British Imperial gallons.
- a historic unit of distance frequently mentioned in the Bible. The word
comes from the Latin cubitum, "elbow," because the unit represents
the length of a man's forearm from his elbow to the tip of his outstretched
middle finger. This distance tends to be about 18 inches or roughly 45 centimeters.
In ancient times, the cubit was usually defined to equal 24 digits
or 6 palms. The Egyptian royal or "long" cubit,
however, was equal to 28 digits or 7 palms. In the English system, the digit
is conventionally identified as 3/4 inch; this makes the ordinary cubit exactly
18 inches (45.72 centimeters). The Roman cubit was shorter, about 44.4 centimeters
(17.5 inches). The ordinary Egyptian cubit was just under 45 centimeters,
and most authorities estimate the royal cubit at about 52.35 centimeters (20.61
- cuerda (cda) 
- a traditional unit of land area in Puerto Rico. The cuerda is equal to about
3930 square meters, 4700 square yards, 0.393 hectare,
or 0.971 acre. Because the cuerda and the acre
are so close to being equal, they are often treated informally as being equal.
Mainlanders sometimes call the unit the "Spanish acre."
- cuerda (cda) 
- a traditional unit of distance in Guatemala, equal to 25 varas
or about 21 meters (roughly 69 feet). Since
cuerda means a cord or rope in Spanish, this unit probably arose as
the length of a measuring rope. The cuerda is also used as an area measure
equal to 1 square cuerda or 625 square varas; this is about 440 square meters
or 527 square yards.
- cuerda (cda) 
- a traditional unit of volume for firewood in Cuba, analogous to the U.S.
cord (see above). A cuerda of firewood is equal to 128 cubic pies, 2.87 cubic
meters, or 0.79 cord.
- an informal unit of flow equal to 1 cubic meter per second or about 35.3147
cubic feet per second. The name was coined after the model of the cusec (see
- cùn, cun, or t'sun
- a traditional unit of length in China, sometimes called the Chinese inch. In modern China, the cùn is equal to exactly 3-1/3 cm, or about 1.312 336 inches. There are 10 cùn in the chǐ or Chinese foot. The cùn is sometimes identified as the distance
between the two outer folds in the bent middle finger, and this personal unit (called the "body inch" in English) is still used by traditional acupuncturists.
- a measure of wood volume used in forestry. One cunit (pronounced cue-nit)
is a volume of timber containing 100 cubic feet (2.8317 cubic meters) of actual
wood (excluding bark and air between the logs). The unit is used mostly for
wood intended as pulpwood or firewood.
- cup (c) 
- a traditional unit of volume used in recipes in the United States. One
cup equals 1/2 (liquid) pint, or 8 fluid
ounces. Technically, one cup equals exactly 14.4375 cubic inches or
approximately 236.6 milliliters, not that anyone measures quite so precisely
in the kitchen. American cooks use the same size cup for measuring both
liquid and dry substances. In Canada, a cup is equal to 8 Imperial fluid
ounces (13.8710 cubic inches or 227.3 milliliters). In Britain, cooks sometimes
used a similar but larger unit called the breakfast
to 10 Imperial fluid ounces. The British cup equals 1/2 Imperial pint, but
the Canadian cup is only 0.4 Imperial pint.
- cup (c) 
- an informal metric unit of volume equal to 250 milliliters, commonly used
in recipes in Australia.
- cup (c) 
- an informal unit of volume for coffee. The size of a cup of coffee varies
according to local custom, but a typical size is about 5 fluid ounces or 150
- curie (Ci)
- a unit of radioactivity. One curie was originally defined as the radioactivity
of one gram of pure radium. In 1953 scientists agreed that the curie would
represent exactly 3.7 x 1010 atomic disintegrations per second,
or 37 gigabecquerels (GBq), this being the best estimate of the activity of
a gram of radium. See also becquerel. The
unit is named for Pierre and Marie Curie, the discoverers of radium.
- a traditional unit of flow equal to 1 cubic foot per second or about 28.317
liters per second.
- cut 
- a traditional unit of length for yarn in Scotland and northern England.
One cut equals 1/12 hank, a unit varying with
the material. A cut of cotton yarn is 70 yards;
a cut of wool is 46 2/3 yards.
- cut 
- a unit of concentration for shellac; see pound
- CV or cv
- a common symbol for the metric horsepower,
standing for the French name cheval vapeur or the Spanish name caballo
- Cwt or cwt
- traditional symbol for the hundredweight.
- a traditional symbol for the cubic yard.
- cycle (c)
- an informal name for "cycle per second." The frequencies of radio signals
and of alternating electric current were previously stated in cycles; thus
the alternating current in American homes is often described as "60-cycle"
and a radio station might describe its signal frequency as "1040 kilocycles",
really meaning 1040 kilocycles per second.
- cycles per second (cps)
- a traditional unit of frequency equal to one per second, or one hertz.
Almost all measurements of frequency are now stated in hertz, the SI
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July 13, 2001