**D**- the Roman numeral 500.
**D-**- an obsolete symbol for the metric prefix deka- or deca-, seen in combinations
such as DL (dekaliter) or DTH (dekatherm). The correct prefix is
**da-**. **dag**- symbol for the dekagram (10 grams; see entry below), an uncommon metric unit of mass.
**daily value (DV)**- a unit of nutrition used in the United States. The U.S. Food and Drug Administration establishes recommended daily amounts of various nutrients, both "good" ones like vitamins and "bad" ones like fat and sodium. These so-called daily values are based on a hypothetical person, male or female, who requires a diet of 2000 Calories per day. The results are approximate at best, since nutritional needs vary with age, sex, and other factors. Food packages generally carry nutritional lables specifying the amount of each nutrient contained in a standard serving, expressed as a percentage of the daily value (%DV). Wikipedia has a table of the official daily values.
**daktylos**- an ancient Greek unit of distance equal to a finger width, about 19 millimeters or 0.75 inch. There were 16 daktylos in the pous, the Greek foot, and 24 in a pechys (cubit). This unit was the Greek predecessor of the Roman digit (see below).
**dal or daL**- symbol for the dekaliter (10 liters; see entry below), an uncommon metric unit of volume.
**dalton (Da or D)**- an alternate name for the unified atomic mass unit (u or amu). The dalton is often used in microbiology and biochemistry to state the masses of large organic molecules; these measurements are typically in kilodaltons (kDa). It seems necessary to have such a unit, since "kilo-amu" would be such a clumsy name. The SI accepts the dalton as an alternate name for the unified atomic mass unit and specifies Da as its proper symbol. The unit honors the English chemist John Dalton (1766-1844), who proposed the atomic theory of matter in 1803.
**dam**- symbol for the dekameter (10 meters; see entry below), a metric unit of distance.
**dan**- a traditional Chinese weight unit, previously spelled
**tan**in many English works. During the European colonial era the unit was equal to 100 catties or 133.333 pounds. In modern China the dan is equal to 100 jin, which is exactly 50 kilograms (110.231 pounds). The dan is the Chinese equivalent of the European quintal or hundredweight. **daN**- symbol for the decanewton or dekanewton (see below under dekanewton), a common metric unit of force.
**Danjon scale**- a scale measuring the brightness (or rather, the darkness) of lunar eclipses, devised by the French astronomer André Louis Danjon (1890-1967). Wikipedia has a description of the scale.
**daraf**- a unit of electrical elastance, which is the ability of an electric potential to charge a capacitor. The daraf is equal to one volt of potential per coulomb of charge (V/C). The name of the unit is "farad" spelled backwards, because the elastance, in darafs, is 1 divided by the capacitance in farads. This unit is not recognized as part of the SI.
**darcy**- a CGS unit of permeability. Permeability is the
extent to which a solid allows the flow of a fluid. This flow depends on the
properties of the solid and also on the dynamic viscosity of the fluid and
the difference in pressure driving the flow. One darcy is the permeability
of a solid through which one cubic centimeter of fluid, having a viscosity
of one centipoise, will flow in one second
through a section one centimeter thick and one square centimeter in cross
section, if the pressure difference between the two sides of the solid is
one atmosphere. It turns out that permeability
has the same units as area; since there is no SI unit
of permeability, square meters are used. One darcy is equal to about 0.98692
x 10
^{-12}square meter. The unit is named for a French scientist, H. Darcy (1803-1858), who did pioneering work in the study of permeability. **darwin**- a logarithmic unit measuring the rate of evolution in characteristics of
organisms. If the measurement of a characteristic (such as length, for example)
changes from
*a*to*b*in*y*million years, then the rate of evolution is (ln*b*- ln*a*)/*y*darwins. (Technically, since the difference in natural logarithms is dimensionless, the darwin is a specialized name for the inverse megayear Ma^{-1}). The unit is named for Charles Darwin (1809-1882), the English biologist who founded the theory of evolution. **dash (ds)**- an informal unit of volume used in food and drink recipes. Originally the dash was usually a liquid measure, small but indefinite in amount, roughly 1/8 teaspoon or a little less. More recently it has been used as both a liquid and dry measure. Kitchen supply stores in the U.S. and other countries have begun selling sets of "minispoons" in which the dash spoon is designed to hold exactly 1/8 teaspoon, which is roughly 0.02 fluid ounce or 0.6 milliliter.
**data mile**- a unit of distance used in radar technology. The data mile equals exactly 6000 ft or 1828.8 meters; this is equivalent to about 1.137 statute (ordinary) miles or 0.9875 nautical miles. U.S. military radar equipment is often calibrated in data miles.
**date**- dates of the year are described in many ways in different languages. To avoid confusion in international communications, the International Organization for Standardization (ISO) adopted in 1986 an international standard (ISO 8601) for representation of dates and times. The ISO 8601 format for dates is YYYY-MM-DD, where YYYY is the year number, MM is the month (01 through 12) and DD the day number. Hyphens, not slashes, must be used as the separators in the representation (if separators are used at all; YYYYMMDD is also acceptable). Thus 2001-04-05 is the representation for the date called April 5, 2001, in traditional U.S. notation. The international standard covers numeric representations only; it doesn't prohibit traditional statements of the date, using words, in whatever the local style is. Thus "April 5, 2009" remains perfectly acceptable in U.S. usage. However, non-standard numeric representations, such as 04/05/09, should not be used, because these representations are interpreted differently in different countries. For additional information, see ISO's Date and Time Format FAQ.
**day [1]**- a traditional unit of time marked by one rotation of the Earth on its axis.
Different cultures have had different ideas about when to mark the start of
the day, whether it should be from one sunset to the next, one sunrise to
the next, etc. (Astronomers sometimes consider the day to run from one noon
to the next, and today we usually mark the day from one midnight to the next.)
This unit is called the
**solar day**, since it is defined by observations of the sun. However, the solar day varies in length, because the speed of the Earth in its orbit varies, with the Earth moving faster when it is closer to the Sun. The result is that solar days are longer during the Northern Hemisphere winter and shorter during the Northern Hemisphere summer, because, despite what Northern Hemisphere residents might think, the Earth is actually closer to the Sun during the Northern Hemisphere winter. Because of this irregularity in the length of actual days, what our clocks attempt to measure is the average period of time between two successive noons, that is, between two successive appearances of the Sun on the meridian (the imaginary line joining due north to due south across the sky). Astronomers call this period of time the**mean solar day**. By long tradition, solar and mean solar days were divided into 24 hours, or 1440 minutes, or 86 400 seconds. **day [2]**- a unit of time generally equal to exactly 24 hours, or 1440 minutes, or
86 400 seconds. This unit is meant to be equivalent (for all practical purposes)
with the mean solar day. But there is a small problem with making this equivalence.
The second has its own scientific definition
based on the frequency of a certain radiation from cesium atoms and having
nothing to do with the Earth's rotation. The reason for making this scientific
definition of the second is that the length of the mean solar day is not constant
from one year to the next. The gravitational attraction of the Moon is very
gradually slowing the Earth's rotation, so that each day is a tiny bit longer,
about 40 nanoseconds on the average, than the previous day. This lengthening
adds one second to the length of the day about every 60 000 years. During
the first decades of the 21st century the actual length of the mean solar
day will be about 86 400.002 seconds. For this reason, time as kept by our
best clocks runs a bit faster than time as kept by the Earth. To keep clock
time and sun time in step (within 0.9 seconds), every so often a
**leap second**must be added at the end of a day. This is done at midnight Universal Time either on June 30 or on December 31. Because the rate of slowing is erratic, the need for these leap seconds cannot be predicted more than a few months in advance. (**Link**:**explanation of leap seconds**from the U.S. Naval Observatory.) **day [3]**- a civil unit of time, the period during which all times have officially the same date (see above). Like day [2], this unit is generally equal to exactly 24 hours, or 1440 minutes, or 86 400 seconds, with a rare extension by addition of a leap second. However, changes in time reckoning also affect the length of the civil day. A day on which "summer" or "daylight" time is adopted has only 23 hours, and a day on which the reckoning reverts to "winter" or "standard" time has 25 hours. Similar adjustments may occur if a locality switches from one time zone to a different zone.
**dB -**- a symbol indicating that a measurement is made using a logarithmic scale
similar to that of the decibel (see below) in that a difference of 10 dB-
corresponds to a factor of 10. In each case, the actual measurement
*a*is compared to a fixed reference level*r*and the "decibel" value is defined to be 10 log_{10}(*a*/*r*). Many units of this kind have been used and only a few of the more common ones are mentioned in the next entries. In each case the dB symbol is followed by a second symbol identifying the specific measurement. Often the two symbols are not separated (as in "dBA"), but the Audio Engineering Society recommends that a space be used (as in "dB A"). **dB A, dB C**- units of sound intensity, exactly like the decibel except that before the measurement is made sounds of high and low frequencies, heard poorly or not at all by the human ear, have been filtered out. The letters A and C refer to two filtering methods.
**dB c**- a unit of signal strength used in electronics, especially in measuring noise
levels. The signals are measured relative to the strength of the carrier signal,
which is the desired signal. A typical statement might be that a certain noise
level is -50 dB c, meaning that the noise is 50 "decibels below carrier" or
10
^{-5}times the carrier signal strength. **dB FS**- abbreviation for "decibels full scale," a unit of power as measured by a
digital device. A digital measurement has a maximum value M depending on the
number of bits used. If the actual power measurement is
*p*, the dB FS value displayed is 20·log_{10}(*p*/M) dB FS. Since*p*cannot exceed M, this reading is always negative. **dB i**- a unit measuring the gain of an antenna. The reference level is the strength
of the signal that would be transmitted by an isotropic antenna: one radiating
equally in all directions. For example, an antenna rated 20 dB i transmits
a signal in the desired direction 10
^{2}= 100 times stronger than an isotropic antenna. **dB m, dB W**- logarithmic units of power used in electronics. These units measure power
in decibels above the reference level of 1 milliwatt in the case of dB m and
1 watt in the case of dB W. A power of
*n*watts equals 10 log*n*dB W; conversely, a power of*p*dB W equals 10^{(}^{p}^{/10)}watts. The same formulas link dB m to milliwatts. An increase of 10 dB m or 10 dB W represents a 10-fold increase in power. Since 1 watt = 1000 milliwatts, 0 dB W = 30 dB m. **dB rn**- a symbol for "decibels above reference noise," a unit measuring noise levels in telecommunications. The usual reference level is -90 dB m, which is equivalent to a power of 1 picowatt (1 pW). For example, 50 dB rn equals -40 dB m.
**dB spl**- a logarithmic unit of sound intensity as computed from the sound pressure
level. The reference level is a pressure of 20 micropascals.
If sound waves exert a pressure of P pascals, the sound intensity is 100 +
20·log
_{10}(P/2) dB spl. **dB u**- a logarithmic unit of power, similar to dB m but computed from voltage measurements.
The reference level is 0.775 volts, the voltage
which generates a power of 1 milliwatt across a circuit having an impedance
of 600 ohms. A voltage of V volts corresponds
to a power of 20·log
_{10}(V/0.775) dB u. **dB V**- a logarithmic unit of power, similar to dB m but computed from voltage measurements.
The reference level is 1 volt. A voltage of
V volts corresponds to a power of 20·log
_{10}(V) dB V. **dB W**- see dB m above.
**dB Z**- a unit of radar reflectivity used in meteorology. The unit measures the amount of energy returned to a weather radar site as a function of the amount transmitted. The scale is logarithmic, a difference of 10 dB Z indicating a 10-fold increase in energy returned. For display purposes, dB Z values are grouped as follows:
- (Level 1, 18-30 dBZ) - Light precipitation
- (Level 2, 30-38 dBZ) - Light to moderate rain

(Level 3, 38-44 dBZ) - Moderate to heavy rain

(Level 4, 44-50 dBZ) - Heavy rain

(Level 5, 50-57 dBZ) - Very heavy rain; hail possible

(Level 6, >57 dBZ) - Very heavy rain and hail; large hail possible - The colorful "radar images" shown on television are actually plots of these levels.
**deadweight ton (dwt)**- a traditional unit of weight or mass used in the shipping industry. The deadweight tonnage of a ship is the difference between its weight when completely empty and its weight when fully loaded. This includes the weight of everything portable carried by the ship: the cargo, fuel, supplies, crew, and passengers. The deadweight ton is traditionally equal to the British ("long") ton of 2240 pounds (1016.047 kilograms). However, more and more often it is being taken to equal the metric ton (exactly 1000 kilograms, or 2204.623 pounds).
**debye (D)**- a CGS unit of electric dipole moment used in chemistry
and physics. A charged molecule can be regarded as a tiny bar with opposite
charges at the ends. Such a bar is called a dipole, and its dipole moment
is the amount of the charge multiplied by half the length of the bar. Thus
the appropriate unit of dipole moment in the SI would
be the coulomb meter (C·m). Since this
is much too large for molecules, the debye is defined as an electric dipole
moment of 10
^{-18}statcoulomb centimeter or 3.33564 x 10^{-30}coulomb meter. The unit honors the Dutch physicist P.J.W. Debye (1884-1966), who was famous for his research on polar molecules. **deca- (da-)**- a metric prefix meaning 10, derived from the Latin word for ten,
*decem*. The Greek form**deka-**is also used, and it has the advantage of avoiding confusion with deci-, one tenth. The International Bureau of Weights and Measures (BIPM) specifies deca- for the prefix and da- for the symbol. In its interpretation of the International System (SI) for U.S. use, the U.S. National Institute of Standards and Technology (NIST) recommends deka- for the spelling of the prefix. Following NIST, this dictionary uses the spelling deka-. **decade [1]**- a traditional unit of quantity equal to 10. Also called the
**decad**. In medieval English this unit was anglicized as the dicker (see below). **decade [2]**- a traditional unit of time equal to 10 years.
**decade [3]**- a unit of time equal to 10 days or 1/3 month. The revolutionary governments in France (1790s) and the Soviet Union (1920s) tried to decimalize and secularize the calendar by eliminating the week (with its traditional day of religious observance) and substituting the decade. Both efforts failed. However, the decade survived as a unit of civil time in Russia and as a 10-day unit in some scientific contexts (for example, a 10-day cycle in meteorology is called a decade). Where the decade equals 1/3 month, the three decades of a month are the 1st-10th, 11th-20th, and 21st through the last day of the month.
**decade [4]**- another name for the bel or the order of magnitude: a logarithmic unit used to compare the sizes of quantities. Two quantities differ by one decade if one is 10 times the other, by two decades if one is 10·10 = 100 times the other, and so on.
**decamillion**- the number 10 million (10
^{7}). **decatherm**- see dekatherm, below.
**decay time [1]**- a unit of relative time used in physics. Decay time is similar to half
life, but shorter and less familiar. It is the time required for an exponentially
decaying process (such as radioactivity) to decrease to 1/e = 36.7879% of
its original value. The fraction of activity remaining at time T, if T is
measured in decay time units, is simply e
^{-T}. The decay time equals 0.693 147 half life. **decay time [2]**- a unit of relative time used in various engineering applications. In many cases, it is the time required for a decaying process to decrease to 10% of its original value. However, a variety of definitions are used in different fields.
**decennium**- a unit of time equal to 10 years; another name for a decade [2]. The census in the U.S. is called decennial because it is taken every ten years.
**deci- (d-)**- a metric prefix meaning one tenth, or 0.1. The prefix is derived from the
Latin word
*decimus*for a tenth. **decibar (dbar)**- a metric unit of pressure equal to 0.1 bar
or 10 kilopascals. One decibar equals 75.006
torr, or 1.450 pounds per square inch (lbf/in
^{2}or psi). The decibar is often used to measure the pressure of seawater, because an increase of 1 decibar in pressure corresponds closely to an increase of 1 meter in depth. **decibel (dB)**- a customary logarithmic measure most commonly used (in various ways) for
measuring sound. The human ear is capable of detecting an enormous range
of sound intensities. Because of this great range, and because our perception
of sound is not linear, it makes sense to measure sound on logarithmic
scales. Informally, if one sound is 1 bel (10
decibels) "louder" than
another, this means the louder sound is 10 times louder than the fainter
one. A difference of 20 decibels corresponds to an increase of 10 x 10 or
100 times in intensity. The beginning of the scale, 0 decibels, can be set
in different ways, depending on exactly which aspect of sound is being
measured. For sound intensity (the power of the sound waves per unit of
area) 0 decibels is equal to 1 picowatt
per square meter; this corresponds approximately to the faintest sound that
can be detected by a person who has good hearing. A quiet room has a normal
sound intensity of around 40 decibels, ten thousand times louder than the
faintest perceptible sound, and a nearby thunderclap may have an intensity of 120
decibels, a trillion times louder than the faintest sound. For sound pressure
(the pressure exerted by the sound waves) 0 decibels equals 20 micropascals
(µPa) RMS, and for sound power 0 decibels
sometimes equals 1 picowatt. In all cases, one decibel equals about 0.115
129 neper and
*d*decibels equal*d*(ln 10)/20 nepers. See also dB- (above). **decigram (dg)**- a metric unit of mass equal to 100 milligrams or about 1.5432 grains.
**decile**- a statistical unit equal to 10 percentiles, or 1/10 of a ranked sample.
See percentile. The word is pronounced
with a soft "c":
*des-ile*. **deciliter (dl or dL)**- a fairly common metric unit of volume equal to 0.1 liter or 100 cubic centimeters. A deciliter contains 6.10237 cubic inches, 3.38140 U.S. fluid ounces, or 3.519 British fluid ounces. The deciliter is similar in size to the gill, an old English unit of volume. The deciliter is commonly used in medicine to express blood volume in units of concentration, such as micrograms per deciliter (µg/dL).
**decimal foot**- an informal term, sometimes used in surveying and construction in the United States. The decimal foot is the same as an ordinary foot, but it is divided decimally instead of being divided into 12 inches. 1 inch is equal to 1/12 = 0.0833333... ft.
**decimeter (dm)**- a fairly common metric unit of distance equal to 10 centimeters or 3.9370 inches. The decimeter is very close to the hand, a traditional English unit.
**decimillimeter (dmm)**- a metric unit of distance equal to 0.1 millimeter (10
^{-4}meter) or about 3.937 mils. The unit is used in civil engineering for stating the results of penetration tests of asphalt concrete, in which a needle is pushed into the concrete under specified conditions. Although this unit is allowed by some standards agencies, the use of compound prefixes such as decimilli- is not permitted in the SI. **decipascal second (dPa·s)**- a unit of dynamic viscosity equal to 0.1 pascal second (Pa·s) or 1 poise. This rather clumsy SI unit is occasionally used because of its equivalence with the poise, an older unit not allowed in the SI.
**decipol**- an empirical unit of indoor odor intensity introduced by the Danish environmental scientist P.O. Fanger in 1988. One olf is defined as the indoor odor intensity produced by one "standard person", and one decipol is the perceived odor intensity level in a space having an odor source of strength one olf and ventilation at the rate of 10 liters/second with unpolluted air. Measurements are recorded by human observers using protocols laid out by Fanger and his colleagues.
**decitex (dtex)**- a common metric unit of yarn density equal to 0.1 tex,
0.9 denier (see below), or 0.1 milligram per meter. This unit was previously
called the
**drex**. **decitonne (dt or dtn)**- a metric unit of mass or weight equal to 100 kilograms (approximately 220.4623 pounds). This unit is becoming common in international trade; it is the same as the Russian centner, the German doppelzentner (see below), and the French metric quintal.
**degree (° or deg) [1]**- the standard unit of angle measure, equal to 1/360 circle, 60 minutes, 3600 seconds, or about 0.017 453 293 radian. So far as we know, this unit was introduced by the Greek geometer Hipparchus of Nicaea (ca. 180-ca. 125 BC), who developed the first trigonometric tables.
**degree (° or deg) [2]**- a unit of distance sometimes used at sea, equal to 60 nautical miles, approximately 69.05 statute miles, or 111.12 kilometers. This distance is the average length of one degree of latitude (that is, the average distance between two lines of latitude 1° degree apart). Because the Earth is not exactly spherical, a degree of latitude actually varies from about 68.7 miles at the Equator to 69.4 miles at the poles. One degree of longitude is about 69.17 miles at the Equator (shrinking to nothing at the poles!).
**degree (° or deg) [3]**- any of the various units of temperature (see below). In the U.S., the unmodified unit "degree" generally means the degree Fahrenheit; in other countries, it generally means the degree Celsius.
**degree (° or deg) [4]**- a unit measuring the hardness of water. Water is called "hard" if it contains
a high concentration of mineral salts, especially calcium carbonate. This
concentration can be expressed clearly in parts per million (ppm) or in milligrams
per liter (mg/L). But starting in the mid-19th century, several uses of "degree"
for water hardness became well established. In the U.S. and Britain, hardness
is measured in
**Clark degrees**, named for the scientist who devised the first reliable test for water hardness. The Clark degree is defined as 1 part of calcium carbonate per 70 000 parts of water; this is about 14.3 parts per million (ppm), 17.1 mg/L, or 1 grain per gallon (gpg, another popular measure of hardness). The French, properly decimal as always, used a degree equal to exactly 10 ppm, while the German degree was equivalent to 17.8 ppm. **degree****(° or deg) [****5]**- various scientific quantities, especially specific gravity and viscosity, have been measured in "degrees" based on the readings of particular instruments. The degree scales of Baumé (see below) and Beck, and many others, have been used in measuring specific gravity, and the scales of Engler and MacMichael are common in the measurement of viscosity. Often these degree scales can't be converted in a simple way to the quantities being measured; one needs the conversion formula or table for the particular instrument. Some of this data can be found in scientific and engineering handbooks, especially older ones.
**degree (****° or deg) [****6]**- the percentage of alcohol, by volume, present in a mixture. In winemaking,
for example, a 13° wine is 13% alcohol by volume (13% v/v). This unit
is also called the
**degree Gay-Lussac**(° GL) after the French chemist Joseph-Louis Gay-Lussac (1778-1850). **degree (deg) [7]**- a unit used in mathematics to describe algebraic expressions. The degree
of an expression having a single variable is the highest exponent (or power)
to which that variable appears. If the expression has more than one variable,
then the degree is the highest total exponent of the individual terms, where
in each term the exponents of the different variables are added. Thus x
^{2}y^{4}z^{3}is a term of degree 9. **degree (****°)****[8]**- the DIN speed rating for film; see DIN below.
**degree (deg) [9] (degree of consanguinity)**- a unit measuring consanguinity: the extent to which two persons are related
to one another by common descent ("by blood," as the usual saying is). Essentially
all cultures, societies, and nations have prohibited marriage between closely
related individuals, and the determining factor is often the number of
degrees of consanguinity between the two persons. If X is an ancestor of
Y, the number of degrees of consanguinity between X and Y is the number
of descents; thus parent and child are separated by 1 degree, grandparent
and grandchild by 2 degrees, and so on. If persons X and Y are not linked
by direct descent but do share a common ancestor A, the usual definition
is that the number of degrees of consanguinity between X and Y is the total
number of descents, counting up from X to A and then back down from A to
Y. For example, siblings (brother and sister) are separated by 2 degrees,
aunt and nephew by 3 degrees, and first cousins by 4 degrees. This calculation
is backed by modern science, because we have learned that persons separated
by
*n*degrees of consanguinity share about 1/2^{n-}^{1}of their genes if they have two common ancestors or 1/2^{n}if they have only one common ancestor. See also cousins. **degree (°) [10]**- a measure of curvature of railroad tracks or highways, used primarily in the U.S. When railroad tracks follow an arc of a circle, the angle of curvature is the angle (measured at the center of the circle) spanned by a chord of a standard length. In the U.S., the chord used is the U.S. chain of 100 feet (30.48 meters). In the metric world, a chord of 20 meters is sometimes used. On most U.S. rail lines curvatures are less than 6° (making the radius greater than 955 feet or 291 meters); on high speed lines they are less than 2° (making the radius greater than 2865 feet or 873 meters). For highways in the U.S., the angle of curvature is the angle (measured at the center of the circle) spanned by an arc 100 ft long.
**degree API (°API)**- a unit invented by the American Petroleum Institute (API) to measure the specific gravity, or one might say the "lightness," of petroleum. Lighter oils are more valuable, so the API scale gives them higher readings than heavier oils. If S is the specific gravity of the petroleum at temperature 60 °F (15.56 °C), the API degree rating is equal to (141.5/S) - 131.5 degrees.
**degree Balling**- see degree Plato, below.
**degree Baumé (°B or °Bé)**- a unit of relative density, as read on a type of hydrometer invented by the French chemist Antoine Baumé (1728-1804). Two scales are used, depending on whether the liquid is lighter than water or heavier than water. For lighter liquids, the relative density d in degrees Baumé is related to specific gravity S by the formula d = (144.3 / S) - 144.3; for heavier liquids the formula is d = 144.3 - (144.3 / S).
**degree Brix (°Bx)**- see brix.
**degree Celsius (°C)**- a metric unit of temperature. The Celsius temperature scale is named for the Swedish astronomer and physicist Anders Celsius (1701-1744), who used a similar scale. The freezing point of water (at one atmosphere of pressure) was originally defined to be 0 °C, while the boiling point is 100 °C. Thus the Celsius degree is 1/100 of the difference between these two temperatures. (The scale actually used by Celsius was inverted, so that 0 °C was the boiling point of water and 100 °C the freezing point.) In the SI system, the Celsius scale is defined so that the temperature of the triple point of water (the temperature at which water can exist simultaneously in the gaseous, liquid, and solid states) is exactly 0.01 °C, and the size of the degree is 1/273.16 of the difference between this temperature and absolute zero (the temperature at which all molecular motion ceases). For practical purposes this is equivalent to the original definition. See also degree Fahrenheit, kelvin.
**degree centigrade (°C)**- an older name for the degree Celsius. The Celsius scale is called "centigrade" because it has 100 (centi-) gradations between the freezing point and boiling point of water. About 1850 scientists began calling the unit "degree Celsius" in honor of its (presumed) inventor.
**degree day (deg da)**- a unit used in meteorology and engineering to measure the demand for heating
or cooling. In the United States, it's agreed that 65 °F (18.3 °C)
is the critical outside temperature; below this tempeature heating is needed
and above it cooling is needed. The number of degree days recorded on a given
date is equal to the difference between 65 and the mean temperature for that
date (figured as the average of the highest and lowest temperatures). If the
mean temperature is above 65,
**cooling degree days (CDD)**are recorded; if it is below 65,**heating degree days (HDD)**are recorded. The total number of heating or cooling degree days recorded in any period of time measures rather accurately the demand during that time for fuel used in heating and cooling buildings. **degree Dornic (°D)**- a measure of the acidity of milk. Each degree Dornic is equivalent to 0.01% lactic acid in the milk. Normal values are around 15.
**degree EBC (°EBC)**- a unit used in Europe to measure the color of beer. EBC is an abbreviation for European Brewing Convention. EBC degrees are related to the Lovibond degrees used in the U.S. (see below) by the approximate formula °EBC = (°L * 2.65) - 1.2.
**degree Fahrenheit (°F)**- a traditional unit of temperature still used customarily in the United States. The unit was defined by the German physicist Daniel G. Fahrenheit (1686-1736), who also invented the mercury thermometer. Fahrenheit set 0° at the coldest temperature he could conveniently achieve using an ice and salt mixture, and it is said he intended to set 96° as the temperature of the human body. (If so, he was off a little there; normal oral temperature for humans is between 98 °F and 99 °F.) On this scale, the freezing point of water (at normal sea level atmospheric pressure) turned out to be about 32 °F and the boiling point about 212 °F. Eventually the scale was precisely defined by these two temperatures. 1°F equals 5/9 °C, but in converting between scales we have to be careful to adjust the zero points as well. To convert a temperature in °F to the Celsius scale, we must first subtract 32° and then multiply by 5/9. In the other direction, to convert a temperature in °C to the Fahrenheit scale, we must first multiply by 9/5 and then add 32°.
**degree Gay-Lussac (°GL)**- see degree [6] above.
**degree Kelvin (°K)**- an obsolete name for the kelvin. In the International System, temperatures on the absolute temperature scale are stated in kelvins, not in degrees Kelvin.
**degree KMW (****°KMW)**- a unit used in Austria to measure the sugar content of must, the unfermented liquor from which wine is made. One degree KMW is roughly equivalent to 1% sugar by weight or 5° Oe; for the exact conversion see below under degree Oeschle. KMW is an abbreviation for Klosterneuburger Mostwaage (Klosterneuburg Must Scale).
**degree Lovibond (****°L)**- a unit used in the U.S. to measure the color (really the darkness) of beer and honey. The scale is open-ended, but most readings fall between 1 (a very light gold, or yellow) and 25 (a very dark brown). A good description of the scale, with comments on other units in use, has been posted by pdlab.com.
**degree MacMichael (°McM)**- a unit used to measure the viscosity, or thickness, of chocolate. Typical values range from around 60 °McM (very thin chocolates suitable for pouring into molds) to around 190 °McM (very thick chocolates suitable for hand dipping or forming around a center). A MacMichael viscometer is used to make the measurement.
**degree Oechsle (****°Oe)**- a unit used in Germany and Switzerland to measure the sugar content of must, the unfermented liquour from which wine is made. One degree Oechsle (or Öchsle) is roughly equivalent to 0.2% sugar by weight. This unit is related legally to °KMW by the formula °Oe = °KMW * ([.022 * °KMW] + 4.54).
**degree Plato (°P)**- a unit measuring sugar content, especially of the wort, the unfermented
liquour from which beer is made. Named for a German chemist, one degree Plato
represents a sugar content equivalent to 1% sucrose by weight. Not all the
sugar in a wort is sucrose; the unit standardizes the measurement to the sucrose
equivalent. The reading is made with a device called a saccharometer. The
**degree Balling**is a somewhat older unit equivalent (approximately) to the degree Plato. In Europe, beer is often taxed either by the degree Plato or by the actual alcohol content. There is no precise conversion between these quantities, but for tax purposes it is often assumed that 1% alcohol (1 degree [6], see above) is equivalent to 2.5 degrees Plato; that is, 1 degree Plato is legally equivalent to 0.4% alcohol. **degree Quevenne (°Q)**- a unit measuring the density of milk. 1 degree Quevenne represents a difference in specific gravity of 0.001, so, for example, 20 °Q milk has specific density 1.020.
**degree Rankine (°R)**- a traditional unit of absolute temperature. 1 degree Rankine represents the same temperature difference as 1 degree Fahrenheit, but the zero point of the scale is set at absolute zero. This means the Rankine temperature is 459.67° plus the Fahrenheit temperature. 1 degree Rankine is equal to exactly 5/9 kelvin. The unit is named for the British physicist and engineer William Rankine (1820-1872).
**degree Réaumur (°r or °R)**- a unit of temperature formerly used in continental Europe. The Réaumur temperature scale is named for the French scientist René-Antoine Ferchault de Réaumur (1683-1757). It is similar to the Celsius scale in having its zero point at the freezing temperature of water, but the interval between the freezing and boiling temperatures of water equals 80 °r instead of 100 °C. Therefore, 1 °r equals 1.25 °C or 2.25 °F.
**degree Soxhlet-Henkel (°SH)**- a measure of the acidity of milk. Each degree Soxhlet-Henkel is equivalent to 0.0225% lactic acid in the milk. Normal values are around 7.
**degree Therner (°Th)**- a measure of the acidity of milk. Each degree Therner is equivalent to 0.009% lactic acid in the milk. Normal values are around 17.
**degree Twaddle (°Tw)**- a unit measuring the specific gravity of liquids denser than water. 1 degree Twaddle represents a difference in specific gravity of 0.005 or 1/200, so a liquid of specific gravity S is measured at 200(S - 1) °Tw. For milk, 1 degree Twaddle equals 5 degrees Quevenne (see above).
**deka- (da-)**- a metric prefix meaning 10, taken directly from the Greek word for ten,
*deka*. The Latin spelling deca- is also used (see deca-, above). Although the U.S. National Institute for Standards and Technology recommends deka-, deca- is seen frequently in American publications. The spelling deca- is also used in French and in other languages where "k" rarely occurs in the middle of a word. Regardless of spelling, da- is the official symbol. (The symbol dk- for deka- is incorrect, but it is seen fairly often.) **dekagram or decagram (dag)**- a common metric unit of mass, the dekagram is frequently used in European
food recipes. One dekagram is equal to 10 grams, 0.01 kilogram, or 0.352 739
66 ounce. The symbol
**dkg**sometimes used for this unit is incorrect. **dekaliter or decaliter (daL or dal)**- a metric unit of volume equal to 10 liters
and comparable to the English peck. The dekaliter
is equal to about 2.641 72 U.S. liquid gallons,
1.135 10 U.S. pecks, or 2.199 69 British Imperial gallons (1.099 85 British
pecks). The symbol
**dkL**sometimes used for this unit is incorrect. **dekameter or decameter (dam)**- a common metric unit of distance equal to 10 meters (about 32.8084 feet).
The symbol
**dkm**sometimes used for this unit is incorrect. **dekanewton or decanewton (daN)**- a fairly common metric unit of force equal to 10 newtons. The dekanewton is equal to 1 megadyne, to 1.019 716 kilograms of force (kgf) or kiloponds (kp), to 2.248 09 pounds of force (lbf), and to 72.3301 poundals. In engineering, the dekanewton is a convenient substitute for the kilogram of force or kilopond, since it is nearly equal to those units.
**dekan**- a unit of angle measure equal to 10° or 1/36 circle. The ancient Egyptians divided the circle of the Zodiac into 36 divisions, which the Greeks called dekans. The unit is still used occasionally in astrology, where one dekan equals 1/3 sign.
**dekare, dekar, or decare**- a metric unit of area equal to 10 ares, that is, 1000 square meters or 0.1 hectare. In English units, the dekare equals approximately 10 763.91 square feet, 1195.99 square yards, or 0.247 105 acres. Various traditional units of land area have been identified with the dekare, including the Middle Eastern dunum (see below), the Norwegian mål, the Greek stremma, and the Vietnamese cong.
**dekatherm or decatherm (DTH)**- a unit of energy equal to 10 therms, 1 million Btu, or about 1.055 057 gigajoules (GJ). This unit is used in the energy industry as a synonym for the million-Btu (MM Btu).
**demal (D)**- an obsolete unit of concentration in chemistry. From 1901 to 1964, the liter was officially defined to be exactly 1.000 028 cubic decimeter. During this period, there was a small difference between measuring concentration in moles per liter and in moles per cubic decimeter. Concentration in moles per liter is called molar, while concentration in moles per cubic decimeter was called demal. This distinction has now disappeared along with the awkward definition of the liter. The former conversion was 1 demal = 1.000 028 molar.
**demi-**- a traditional prefix meaning 1/2. The prefix is derived from the Latin
*dimedius*, meaning a cut in the middle of something. In the original (1795) definition of the metric system, this prefix was allowed for the weight and volume units such as the gram, kilogram, and liter, but not for the length units. These uses have been obsolete for many years. **demi [1]**- a half bottle of wine (375 milliliters).
**demi [2]**- an informal French unit of volume for beer, generally equal to 250 milliliters
(1/4 liter). The unit was originally a half pint (
*demipinte*). **demisemiquaver**- a unit of relative time in music equal to 1/32 whole note or 1/64 breve.
**denaro**- a traditional Italian weight unit equal to 24 grani; its size varied from about 1.1 grams to 1.25 grams.
**denier [1]**- a traditional unit of yarn density. One denier is the density of a thread having a mass of 1 gram per 9 kilometers of length. The metric unit of yarn density is the tex; 1 denier equals 1/9 tex or 10/9 decitex.
**denier [2]**- a traditional French weight unit, comparable to the English pennyweight.
Like the pennyweight, the denier is equal to 24 grains
(1.275 grams). This unit was also called the
**scrupule**. **desiccant unit (DU)**- a unit measuring the amount of a drying agent. One desiccant unit is the amount of the drying agent that can absorb 3 grams of water at a relative humidity of 20% and 6 grams at a relative humidity of 40%, when the temperature is between 21 °C (69.8 °F) and 25 °C (77 °F). (Different industrial standards differ slightly in the temperature specification.)
**dessertspoon or dessertspoonful (dsp or dssp)**- a unit of volume sometimes used in food recipes. The dessertspoon is equal to 2 teaspoons; this is roughly equivalent to 10 milliliters in the U.S. In the metric world, a measuring spoon holding exactly 10 milliliters is often called a dessertspoon.
**dessiatina**- a traditional unit of land area in Russia equal to 2400 square sadzhens. By coincidence, this makes the dessiatina very nearly the same as a hectare: it equals about 1.0925 hectare or 2.6996 acres.
**deuce**- an old English word for two, derived from the old French
*deus*(now spelled*deux*). The word survives as the name for a two-spot showing in dice or a two card in card games. In tennis, "deuce" describes a tie situation in which a player must win the next two points in order to win the game. **devil's dozen**- an alternate name for the baker's dozen, a traditional unit of quantity equal to 13. This name reflects the long-standing association of the number 13 with bad luck or evil.
**dex**- a logarithmic unit being used in astronomy. Originally, dex was a convenient
function defined by dex (
*x*) = 10. But the notation is now being used after the exponent in expressions such as -.043 dex, meaning 10^{x}^{-.043}. Thus 1 dex equals a factor of 10, making the dex identical to the bel. The name "dex" is a contraction of "decimal exponent." **dhur**- a traditional unit of land area in South Asia, equal to 1/20 kattha or 1/400 bigha. Like the bigha, the dhur varied in size from region to another. In Nepal, where the unit is still in use, the dhur equals about 16.9 square meters or 20.2 square yards.
**dialog unit (dlu)**- a unit of relative distance used in computer graphics. Actually there are two units: the horizontal dialog unit equals 1/4 the average width of the font being used, and the vertical dialog unit equals 1/8 the average height of the font. If the font's aspect ratio (the ratio of height to width) is 2:1, these two units will be the same. This is often the case. The unit is used particularly in the design of dialog boxes.
**diameter (dia, X, or ×)**- a unit of magnification equivalent to power. "Power" tends to be used in adjective phrases, such as "120-power eyepiece" or "7-power binoculars." "Diameters" often appear in declarative statements of magnification, such as, "the eyepiece lens magnifies 120 diameters."
**diamond**- a mysterious marking on many tape measures in the U.S. The diamonds mark a distance unit equal to exactly 8/5 feet (19.2 inches or 48.768 centimeters). This is potentially useful to carpenters if they wish to place 5 studs, floor joists, etc., in a distance of 8 feet.
**dicker**- a traditional unit of quantity equal to 10. The word is an English version
of the Latin word for ten,
*decem*. After centuries of use, especially by traders in furs and skins, the dicker has nearly disappeared from the English language. **Didot point**- a unit of distance used in typography; see point [2].
**dieb. alt., dieb. tert.**- abbreviations for the Latin
*diebus alternis*, every other day, and*diebus tertius*, every third day, units of frequency sometimes used in medical prescriptions. **digit**- a historic unit of distance equal to the width of a person's finger. Used
in all the ancient civilizations of the Middle East and Mediterranean, the
digit was equal to 0.75 inch or 19 millimeters with only the smallest variations.
Typically, there were 4 digits in a palm, 16
in a foot, and 24 (sometimes 28) in a cubit.
The word digit is from the Latin word for a finger or toe,
*digitus*. **dimension (dim)**- a mathematical unit measuring the number of independent directions in a set or space. Traditionally, a space has as many dimensions as there are mutually perpendicular directions at each point in the space: thus a line has 1 dimensions, a plane has 2 dimensions, and the ordinary space we live in has 3 dimensions. The theory of relativity is set in a "space-time" having 4 dimensions, and higher dimensional spaces are frequently used in science and economics. In addition, mathematicians have developed several methods for assigning fractional dimensions to certain complex sets.
**DIN**- the initials of Deutsches Institut für Normung, the German standards agency. Outside the U.S., film was generally marked with a DIN speed rating such as DIN 24 (corresponding to ASA 200). A difference of 3 in the DIN rating corresponds to a doubling of the film speed; that is, DIN 27 film (ASA 400) is twice as fast as DIN 24. The ASA and DIN ratings are now combined, with a degree symbol on the DIN number, and marked with ISO (the initials of the International Organization for Standardization). Thus DIN 27 film is now marked ISO 400/27°.
**diopter (dpt or D)**- a metric unit used in optics to measure the refractive power of a lens.
Each converging lens has a focal length, defined to be the distance from the
center of the lens to the point at which the lens focuses light. (Diverging
lenses, which can be used to convert focused light to parallel rays, have
a negative focal length.) The shorter the focal length, the greater the refractive
power of the lens. The refractive power of the lens, in diopters, equals 1
divided by the focal length of the lens, in meters, so 1 diopter = 1 m
^{-1}. The unit is often spelled*dioptre*outside the United States. The lower case Greek letter delta is also used as a symbol for the diopter. See also prism diopter. **diraa**- a traditional Egyptian unit of distance equal to about 58 centimeters (22.8 inches).
**djerib**- see jerib.
**dk-**- an incorrect symbol for the metric prefix deka- (10), sometimes seen in
combinations such as
**dkg**for the dekagram or**dkm**for the dekameter. The correct symbol is**da**-. **DN**- a symbol for "nominal diameter," a size measure for piping, valves, fittings, etc. Nominal diameter is essentially the inside diameter of the piping in millimeters. Industrial standards organizations, such as the American National Standards Institute (ANSI), set standards for pipes and fittings based on DN ratings; these standards specify in detail the size, composition, and strength of each component.
**Dobson unit (DU)**- a unit used in geophysics to measure the ozone in the atmosphere. One Dobson
unit represents the amount of atmospheric ozone that would form a uniform
layer 0.01 millimeter (10 micrometers) thick at standard temperature (0 °C)
and pressure (1 atmosphere or 1013.25
millibars). The Dobson unit equals 10
^{-5}atmo-meter. Under normal conditions, the atmosphere contains about 300 Dobson units of ozone, but this falls to 100 Dobson units or less in the "ozone holes" over the Earth's poles. The unit is named for the British physicist G.M.B. Dobson; in 1920 he invented a spectrometer to measure ozone concentrations from the ground. **dog watch**- a unit of time on ships at sea equal to 2 hours, one half the usual length of a watch.
**dog year**- an informal unit of time equal to 1/7 of a normal, or "human" year. According to folklore, dogs age 7 times faster than humans.
**dol**- a unit proposed for the measurement of pain. James Hardy, Herbert Wolff,
and Helen Goodell, all of Cornell University, proposed the unit based on their
studies of pain during the 1940s and 1950s; they defined one dol to equal
2 "just noticeable differences" (jnd's) in pain. However, the unit did not
come into widespread use and other methods are now used to assess the level
of pain experienced by patients. The name of the unit is from the Latin word
for pain,
*dolor*. **dollar ($)**- a unit used in nuclear engineering to describe the "reactivity" of a nuclear
reactor. For a particular reactor, one dollar is the reactivity at which the
chain reaction is just self-sustaining. The size of the unit varies with the
design of the reactor; a typical size is approximately 10
^{-5}= 0.001%. For a discussion of reactivity, see inhour. **donkey power**- a somewhat light-hearted metric unit of power equal to 250 watts or about 0.3353 horsepower.
**donum, dönüm**- see dunum (below).
**doppelzentner (Dz)**- a name often used in Germany for the decitonne or metric quintal. Since a zentner is 50 kilograms, a doppelzentner or "double" zentner equals 100 kilograms (about 220.462 pounds).
**double-**- a traditional prefix meaning 2. In the original (1795) definition of the metric system, this prefix was allowed for the weight and volume units such as the gram, kilogram, and liter, but not for the length units. These uses have been obsolete for many years.
**double**- a traditional unit of volume for liquor equal to 2 shots. In the U.S., a double equals 2 fluid ounces or about 59.15 milliliters.
**double magnum**- a large wine bottle holding about 3 liters, 4 times the volume of a regular
bottle. A double magnum is also called a
**jeroboam**. **doublet**- another name for a pair.
**double word or doubleword**- a unit of information equal to 2 shortwords, 4 bytes or 32 bits. See also word [2].
**douzième**- a traditional unit of distance used in watchmaking. The word is French for "twelfth," and a douzième is equal to 1/12 Swiss ligne. This is about 188 micrometers (microns) or 7.4 mils.
**dozen (doz or dz)**- a familiar unit of quantity equal to 12. Division into units of 12 rather
than 10 has the advantage that 12 can be evenly divided into halves, thirds,
or quarters. For this reason, units of 12 have been common since the earliest
civilizations of the Middle East. "Dozen" comes from an old French word
*dozaine*related to the Latin word*duodecem*, "twelve." One dozen dozen (that is, 144) is called a gross, and one dozen gross is called a great gross. **dpa**- abbreviation for "displacements per atom," a measure of the damage to a crystalline material caused by bombarding the material with energetic particles. Each displacement represents an atom dislodged from its place in the crystal by the radiation.
**dpi**- a unit used to measure the resolution, or sharpness, of a photograph or video image. The unit, an abbreviation for "dots per inch," remains in use even though individual picture elements are now called pixels rather than dots.
**dpm, dps**- abbreviations for "decay per minute" and "decay per second," respectively, units used in the measurement of rates of radioactivity. 1 dps is properly called 1 becquerel (Bq). 1 dpm is equal to 1/60 becquerel or 0.45045 picocuries.
**drachm**- traditional British spelling of dram (see below).
**drachma**- a traditional unit of weight in Greece and the eastern Mediterranean. The drachma was a Greek coin, and the unit was originally the weight of the coin. In ancient times this was about 4.3 grams, but more recent versions are about 3.2 grams, 49.3 grains, or 0.113 ounce avoirdupois. There is also a traditional Dutch weight unit of the same name, but it is somewhat larger: 3.906 grams, 60.28 grains, or 0.1374 ounce avoirdupois. Compared to a similar English unit, the Greek drachma is about 2.5 pennyweight and the Dutch is almost exactly 3 pennyweight.
**drachme**- a traditional unit of weight in France and Germany, generally between 3.7 and 3.9 grams.
**dram (dr) [1]**- a unit of weight in the traditional avoirdupois
system (see pound [1]), equal to 1/16
ounce or 1/256 pound. One dram equals about 1.7718 gram.
The word dram comes from a Latin weight unit, the
*dragma*, and derives ultimately from the Greek*drachme*, meaning a handful. The word is usually spelled "drachm" in Britain and "dram" in the United States, but both spellings are pronounced "dram." The avoirdupois dram is sometimes abbreviated**dr. av.**to distinguish it from the apothecaries' dram (next entry). **dram (dr) [2]**- a unit of weight in the traditional system of English apothecaries, equal
to 60 grains, 3 scruples, 1/8 troy ounce, or
approximately 3.8879 gram. See troy
weights for additional information. The apothecaries' dram is sometimes
abbreviated
**dr. ap.**to distinguish it from the avoirdupois dram. It is equivalent to about 2.1943 avoirdupois drams. There is a similar Italian unit, the**dramma**, equal to 72 grani or roughly 3.5 grams. **dram (dr) [3]**- a traditional unit of volume. See fluid dram.
**drex**- a unit traditionally used in the textile industry to measure the density
of a single fiber of yarn. One drex equals a density of one gram per 10 kilometers
of length, or 1 microgram per centimeter (µg/cm). Since 1 drex equals
0.1 tex, the unit is now called the
**decitex (dtex)**. **drill sizes**- traditional drill sizes are numbers 1-80, with larger numbers indicating smaller drills. Number 1 has diameter 0.2280 inch and number 80 has diameter 0.0135 inch. Larger sizes are designated by letters or by specifying the diameter directly in 64ths of an inch. The metric drill size is the diameter in millimeters. EngineersEdge.com has a table showing the traditional sizes and metric equivalents.
**drink**- a unit measuring the alcohol content of beverages, used in describing the medical effects of alcohol. U.S. physicians generally consider one drink equal to 0.5 U.S. fluid ounces of alcohol; the appropriate metric equivalent would be 15 milliliters. In U.S. fluid units, one drink corresponds to about 4 ounces of wine, 10-12 ounces of beer, or 1.25 ounces of whiskey.
**drap**- a traditional unit of weight in Scotland, equal to 1/16 Scots ounce
or about 1.9 grams. Sometimes spelled
**drop**, this unit was the Scottish counterpart of the English dram [1]. **drop (gtt) [1]**- a unit of volume used in pharmacy. Traditionally the drop was another name
for a minim, a unit of volume equal to 1/60
fluid dram or 1/480 fluid
ounce (about 0.0616 milliliter in the U.S., 0.0592 milliliter in Britain).
Now that prescriptions are written in metric units, the pharmacist's drop
is equal to exactly 0.05 milliliter (20 drops/ml). In hospitals, intravenous
tubing is used to deliver medication in drops of various sizes ranging from
10 drops/ml to 60 drops/ml. The traditional abbreviation is from the Latin
*gutta*, drop. Originally,**gt**was the symbol for a single drop, with**gtt**being the plural. **drop [2]**- a informal unit of volume used in recipes. According to some older kitchen references, 24 drops = 1/4 teaspoon; with U.S. definitions this makes the drop equal to 1/576 fluid ounce or about 0.051 milliliter, comparable to the pharmacist's drop (previous entry).
**drought severity category (D)**- a measure of drought severity developed by the U.S. National Drought Mitigation Center and used widely by other agencies in the U.S. Categories are denoted D0-D5, with higher numbers indicating more severe drought. The Center has a description of the scale.
**drum**- a unit of volume sometimes used in the oil trade. The traditional standard drum of oil contains 55 U.S. gallons or about 208.198 liters. One 55-gallon drum equals 1.3095 barrels [2]. In the metric world, drums of 200 liters, 205 liters, and 208 liters are in use as substitutes for the traditional size.
**dry pint, dry quart**- common names for the U.S. pint and quart units used for dry commodities, used to distinguish these units from the pint and quart used for liquids. The dry units are equal to about 1.163 647 times the corresponding liquid units.
**dry ton**- see wet ton.
**dsp, dssp**- symbols for the dessertspoon (2 teaspoons). See "dessertspoon" above.
**duet or duo**- a traditional unit of quantity equal to 2. The word
*duet*is a French version of the Italian*duo*, two. **dunum, dönüm, or donum**- a traditional unit of land area in the Middle East and the Balkans. The unit is of Turkish origin, dönüm being the Turkish spelling. As it is commonly used today in Turkey, Israel, Palestine and in Croatia and other areas of the former Yugoslavia, the dunum is a metric unit equal to 1000 square meters or 0.1 hectare (about 0.2471 acre). The traditional size seems to have been around 900 square meters. In Mesopotamia and Arabia, the dunum was a larger unit, traditionally in the range of 2500 to 4000 square meters. In modern Iraq, the dunum is now standardized at 2500 square meters (about 0.6179 acre).
**duro, durometer**- a measure of the hardness of plastic, rubber, or similar material, made by an instrument called a durometer. These instruments test the material with a steel pin, and the depth of the indentation made by the pin is read by a calibrated spring meter. There are various versions of the durometer, designed for materials of different kinds; to be called durometers, all versions must conform to standards set by standards organizations such as the American Society for Testing and Materials (ASTM). Durometer scales run from 0 to 100, with larger numbers indicating harder material. The different scales are identified by letters, including A, B, C, D, O, DO, and OO. However, since only one type is used for a particular material, the scale designation is often omitted. Typical notations used are "50 duro", "65 durometer A", and "D40 duro".
**dwt**- abbreviation for deadweight ton (see above); also the traditional symbol for pennyweight.
**dyad**- a unit of quantity equal to 2. This unit is frequently used in cell biology and biotechnology.
**dyne (dyn)**- the CGS unit of force. One dyne is the force that
accelerates a mass of one gram at the rate of one centimeter per second per
second. Expressed in SI units, the dyne equals 10
^{-5}newton. This is quite a small force: it equals about 2.248 x 10^{-6}pound of force (lbf) in the traditional English system. The word dyne comes from the Greek*dynamis*, power.

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August 21, 2001. Revised December 8, 2008.