O
 obol, obolos, obolus
 a historic unit of weight or mass. The obol is a very small weight that
originated as the weight of a tiny Greek coin. In ancient Greece the obolos
was equal to 1/6 drachma, or roughly half a gram (8 grains).
In Rome, the obolus was equal to 1/48 Roman ounce (uncia) or
about 0.57 gram. In modern Greece, the obolos is an informal name for
the decigram (0.1 gram).
 o'clock [1]
 a contraction of the phrase "of the clock," used in English after a statement
of time. This phrase has been traced to the early 1400s at least; it is fairly
common in the works of Shakespeare. Earlier, time was usually stated in hours
and minutes, and this is still the case in most languages. Thus "10 o'clock"
is "10 hours" in most of the world.
 o'clock [2]
 an informal angular measure that works by describing an angle in terms of
the face of a standard (12hour) clock. Each hour "o'clock" spans an angle
of 30°, so "4 o'clock" means an angle of 120° measured clockwise
from dead ahead or some other agreedupon point of reference.
 octad
 a unit of quantity equal to 8.
 octane number or octane rating
 a measure used to express the ability of gasoline to reduce engine knocking.
Gasoline is a complex mixture of hydrocarbons: compounds containing hydrogen
and carbon. Beginning chemistry students learn that "octane" is the name
of a hydrocarbon whose molecules contain 8 carbon atoms and 18 hydrogen
atoms, the 8 carbons being arranged in a long chain. The compound cars
need to prevent knocking is not that octane but a different compound of
8 carbon atoms and 18 hydrogen atoms called isooctane or, in the more
precise language of chemical nomenclature, 2,2,4trimethylpentane. (In
an isooctane molecule there are only 5 carbons in the chain. Carbons 6
and 7 are attached to the sides of the chain at the #2 position, and the
last carbon is hooked onto the #4 position; chemists call this a branched
hydrocarbon.) To determine the octane rating of gasoline, a sample of the
gasoline is compared to a laboratory mixture of isooctane and another
hydrocarbon called heptane (heptane has 7 carbons and 16 hydrogens, with
the 7 carbons in a chain). The mixture is adjusted until it has the same
antiknocking characteristics as the gasoline being tested. The octane
rating is the percentage of isooctane required in the laboratory mixture
to produce this equality of knocking behavior. In fact, it's even more
complicated: there are two ways to do the test, producing two ratings called
the research octane number (RON) and the motor octane number (MON). The
MON is typically 8 or 10 points lower than the RON for the same batch of
fuel. In the U.S., the number posted on the gas pump is the average of
the RON and MON; this average is called the pump octane number (PON). There
is a similar unit, cetane number, used
for rating diesel fuel. The octane rating is often misunderstood as a
measure of the energy content of the fuel, but what it actually measures
is the tendency of the fuel to burn rather than explode.
 octant [1]
 a unit of angle measure equal to 1/8 circle, 45°, or pi/4
radians.
 octant [2]
 a unit of solid angle measure. One octant is 1/8 sphere, or pi/2
steradians, or about 5156.6 square degrees.
 octarius
 an obsolete name for the British Imperial pint
(34.678 cubic inches or approximately 568.261 milliliters), used in British
medicine and pharmacy during the nineteenth century. The octarius was equal
to 1/8 congius (gallon).
 octave
 a unit used in music to describe the ratio in frequency between notes. Two
notes differ by one octave if the higher note has exactly twice the frequency
of the lower one. The name, derived from the Latin numeral 8 (octo),
refers to the traditional sequence of 8 notes in the musical scale (in English:
do, re, mi, fa, so, la, ti, do). Both ends of the scale are included, so 2
octaves include 15 notes (first do through third do) instead of 16. Outside
of music, the octave is also used to describe a group of 8 objects sequenced
somewhat like musical notes. For example, in the Christian religious calendar
an octave is a period of 8 days beginning with a feast day and ending with
the day one week after the feast day.
 octavillo
 a traditional Spanish unit of dry volume. The octavillo equals about 289
milliliters (a little more than a cup, in U.S.
terminology). This is equivalent to about 17.64 cubic inches, 0.525 U.S. pint,
or 0.5086 British Imperial pint. Since octavillo means "eighth," one would
expect the octavillo to be 1/8 of some other unit, but this is not the case.
There are 4 octavillos in a cuartillo,
16 in an almude, and 48 in a fanega.
 octennium
 a unit of time equal to 8 years.
 octet [1]
 a unit of quantity equal to 8. The name comes from the Latin numeral octo.
 octet [2]
 a unit of information equal to 8 bits, used
primarily in telecommunications. In most contexts, this is the same as a byte,
but the byte can sometimes vary in size while the octet is always exactly
8 bits.
 octuple, octuplet
 a group of 8 items, especially 8 identical items; the word octuplet is also
used for one member of the group.
 odds
 a statement of the probabilities that an event will or will not happen.
If the probability is a/(a + b) that the event will happen
and b/(a + b) that it will not happen, then the odds are ab (or a
to b, or a:b) in favor of the event and ba (or b
to a, or b:a) against the event. In the division of the
probability into a
+ b parts the values of a and b are usually chosen so that
both numbers are small whole numbers with the smaller number equal to 1 if
possible. Generally the odds are stated
in whichever form puts the larger number first, and in many cases this will
be the odds against the event. Thus 41 odds on a horse in a horse race can
be viewed as an estimate that the probability of the horse's winning is 1/5.
In some cases, odds are used to express probabilities explicitly: for example,
if a geologist says that the odds are 51 against a volcano erupting this
year, that is an estimate that the probability of an eruption is about 1/6.
In betting situations, odds are at least a rough estimate for probabilities,
but what they actually establish is the payoff ratio: if an event has ab odds,
then you must bet b in order to receive a + b if the
event happens. Thus the odds can be viewed as the ratio of potential profit
a to wager b. This traditional use of the word "odd" is
believed to come from the old English word ord for a spear point,
hence something hazardous.
 Oechsle
 see degree Oechsle.
 oersted (Oe) [1]
 the CGS unit of magnetic field strength. The oersted
is defined to be the field strength in a vacuum at a distance 1 centimeter
from a unit magnetic pole. A field
of one oersted generates a magnetomotive force of 1 gilbert
per centimeter of conductor. There is no named MKS
unit of field intensity, but the oersted is equivalent in MKS units to 79.577
472 ampereturns per meter. The unit honors
Hans Christian Ørsted (17771851), a Danish physicist, who discovered
electromagnetism in 1820. Before 1930 this unit was called the gauss
[2].
 oersted (Oe) [2]
 another name for the gauss [1] as
a unit of magnetic flux density.
 ohm [1]
 the SI unit of electric resistance, reactance, and
impedance. If a conductor connects two locations having different electric
potentials, then a current flows through the conductor. The amount of the
current depends on the potential difference and also on the extent to which
the conductor resists the flow of current. For direct current circuits, this
property of opposition to current flow is called the resistance. In alternating
current circuits, the current flow is also affected by reactive components,
capacitors or inductors, that react to the change in the current over time.
This opposition is called reactance; impedance measures the combined effect
of resistance and reactance. All three quantities are measured in ohms. One
ohm is the resistance, reactance, or impedance that requires a potential difference
of one volt per ampere
of current. The unit honors the German physicist Georg Simon Ohm (17871854).
The capital Greek letter omega is used as the
symbol for the ohm, since "O" would be easily misinterpreted as a zero.
 ohm [2]
 a traditional German unit of liquid volume, no longer in use. The ohm, spelled
aume in English, was generally equal to 1/6 fuder
or roughly 150 liters (40 U.S. gallons).
 ohm meter
 a unit of resistivity, measuring the extent to which a substance offers
resistance to passage of an electric current. The resistivity of a conductor
in ohm meters is defined to be its resistance (in ohms) multiplied by its
crosssectional area (in square meters) divided by its length (in meters).
 ohm per square
 a unit of resistivity for surface films and other materials whose thicknesses
are considered to be negligible. The resistivity of a very thin conductor
is defined to be its resistance (in ohms) multiplied by its width and divided
by its length. If the conductor is square in shape, then its length and width
are the same and its resistivity is numerically equal to the resistance of
the square, which is actually the same no matter what the size of the square
is. Therefore the resistivity could be stated in ohms, but it is conventional
to state it in "ohms per square." One can consider the square to have sides
equal to one unit, the size of the unit being immaterial.
 ogdoad
 another name for an octad; this very learned spelling is a transliteration
of the ancient Greek.
 oitavo
 a traditional Portuguese unit of liquid volume comparable to 1/2 gallon
in the U.S. system. The oitavo equals 1/32 fanega
or about 1.73 liters (0.46 U.S. liquid gallon or 0.38 British Imperial gallon).
 oka or oke
 a traditional unit of weight in Turkey and throughout the eastern Mediterranean.
The oka is approximately 2.8 pounds or 1.28 kilograms, although its size varied
somewhat over the large area formerly included in the Turkish empire. In Greece,
the oka was standardized at 1282 grams and remained in use until traditional
units were prohibited in 1959. The Greek oka was divided into 400 dramja;
in Cyprus, under British rule, the oka was divided into 400 drachms and remained
in use until the 1980s. In Turkey, the oka was redefined in 1931 to be equal to the kilogram. The oka was also used sometimes as a unit of liquid
volume, representing the volume (roughly 1.25 liters) occupied by an oka (weight)
of water or wine.
 okta
 a unit of proportion equal to 1/8, used in meteorology to record the fraction
of the sky covered by clouds. For example, if half the sky is cloudcovered,
the coverage is reported to be 4 oktas. The name of the unit comes from the
Greek numeral 8, okto. It was coined to provide a word meaning "eighth"
in all languages.
 öl
 a traditional Hungarian distance unit comparable to the English fathom.
The öl was equal to 6 láb or about
1.896 meter (6.22 English feet), making it the Hungarian version of the Viennese
klafter.
 old style (OS)
 a notation used after a date stated in the Julian calendar (see year
[2]). The Julian calendar was replaced by the Gregorian calendar
in 1582 or soon thereafter in predominantly Catholic countries of Europe.
It remained in use in Britain, a Protestant country, until 1752, and in Russia,
an Orthodox country, until 1918. In addition, the first day of the new year
in England, until 1752, was 25 March instead of 1 January. Thus George Washington,
the first U.S. president, was born on 11 February 1731 OS, or 22 February
1732 in the Gregorian calendar. Link: adoption
dates for the Gregorian calendar
in various countries. Wikipedia has a useful chart with similar information.
 olf
 an empirical unit of indoor odor intensity introduced by the Danish environmental
scientist P.O. Fanger in 1988. One olf is defined as the odor intensity produced
by one 'standard' person (a standard person is also defined). The name comes
from the Latin olfacere, to smell. Ventilation reduces pollution, and
the resulting pollution in ventilated, enclosed spaces is measured in decipols.
 olk
 a traditional Iraqi unit of land area, now identified with the are,
the metric unit of area equal to 100 square meters. Like the are, the olk
is approximately 1076.3910 square feet or 0.02471 acre. There are 25 olk in
the Iraqi dunum, the common unit of agricultural
land area in the country.
 olympiad
 a unit of time equal to four years. In ancient Greece, the olympiad referred
to the fouryear interval between successive Olympic Games. The first Greek
olympiad was the period 776773 BC. The olympiad was revived in 1896 when
the modern Olympics began. The period 20052008, called the "28th Olympiad
of the modern era," is the 696th olympiad by the original Greek reckoning.
 omer
 a ancient unit of volume for grains and dry commodities, used in the Bible.
The omer was equal to 0.1 ephah; this is believed to equal about 4.032 liters,
246.05 cubic inches, 0.9154 U.S.dry gallon,
or 0.8869 British Imperial gallon.
 omn. bih.
 traditional abbreviation for the Latin omni bihorio, once every two
hours, a unit of frequency sometimes used in medical prescriptions. The abbreviation
alt. h. (alternis horis, every other hour) is equivalent.
 omn. hor.
 traditional abbreviation for the Latin omni hora, once every hour,
a unit of frequency sometimes used in medical prescriptions. The abbreviation
q. h. (quaque hora, each hour) is equivalent.
 onça, once, oncia, onza
 traditional names for the ounce unit in Romance languages. The Portuguese
onça and Spanish onza equal 1/16 libra
or about 28.69 grams (1.012 ounce); the French once equals 1/16 livre
or about 30.59 grams (1.079 ounce). The Italian oncia or onza
is no longer used, but traditionally it equaled 1/12 libra
or about 27.3 grams (0.96 ounce).
 ons
 a Dutch unit of weight or mass, now used as a metric unit equal to the hectogram
(100 grams, or about 3.5274 ounces).
 open window unit (owu)
 the original name of the unit of sound absorption now called the sabin.
 order of magnitude
 a logarithmic unit used to compare the sizes of quantities. Two quantities
differ by one order of magnitude if one is 10 times the other, by two orders
of magnitude if one is 10·10 = 100 times the other, and so on. Thus a
difference of n orders of magnitude means the larger quantity is 10^{n}
times the smaller one.
 osmolal, osmolar
 notations used by chemists to describe the concentration of ions in chemical
solutions. The term "osmolal" describes an ion concentration of a solution
in moles per kilogram of solvent (mol/kg), while "osmolar" describes an ion
concentration in moles per liter (mol/L).

 osmole (Osm)
 a unit of osmotic pressure used in physical chemistry, cell biology, and
medicine. If chemical solutions are separated by a semipermeable membrane
(a membrane that resists the passage of dissolved substances but permits the
passage of the solvent, usually water), then the solvent will diffuse across
the membrane to equalize the concentrations. This process is called osmosis.
Solutions with higher concentrations of dissolved substances are said to have
higher osmotic pressure than solutions having lower concentrations;
thus the solvent moves from an area of low osmotic pressure to an area of
higher osmotic pressure. One osmole is the osmotic pressure of a one molar
solution (that is, a solution with a concentration of one mole
per liter of solvent) of a substance that does not dissociate, such as sugar
(glucose) in water. Osmotic pressure depends on the total number of dissolved
particles, so for a substance that dissociates into two ions, such as ordinary
salt (sodium chloride), a one molar solution has an osmotic pressure of 2
osmoles. In practice, most measurements are in milliosmoles (mOsm). Typical
values range from 20 mOsm for fresh water through 290 mOsm for typical human
blood plasma to 1010 mOsm for salt water from the open ocean.
 ostent
 a medieval name for the time unit now called the minute. (In medieval times
a minute was equal to 1/10 hour, or 6 modern minutes). The ostent was equal
to 8 ounces [4] (see below).
 ounce (oz or oz av) [1]
 a traditional unit of weight. The avoirdupois ounce, the
unit commonly used in the United States, is 1/16 pound or about 28.3495 grams.
The avoirdupois ounce also equals 175/192 = about 0.911 457 troy ounce or
437.5 grains. The word ounce is from the Latin
uncia, meaning a 1/12 part, because the Roman pound was divided into
12 ounces. The word "inch," meaning 1/12 foot, has the same root. The symbol
oz is from the old Italian word onza (now spelled oncia)
for an ounce. See avoirdupois weights
for additional information.
 ounce (oz, oz t, toz, or oz ap) [2]
 a second traditional unit of mass or weight. The troy ounce,
traditionally used in pharmacy and jewelry, is 1/12 troy pound, 480 grains,
or about 31.1035 grams. Thus the troy ounce equals 192/175 = 1.09714 avoirdupois
ounces. This unit is the traditional measure for gold and other precious metals;
in particular, the prices of gold and silver quoted in financial markets are
the prices per troy ounce. The troy ounce is divided into 20 pennyweight
or into 8 troy drams [2]. See troy
weights for additional information. The troy ounce is sometimes abbreviated
oz t or toz to distinguish it from the more common avoirdupois ounce; in traditional
pharmacy it was abbreviated oz ap.
 ounce (oz or fl oz) [3]
 a traditional unit of liquid volume, also called the fluid
ounce (fl oz).
 ounce [4]
 an old term for a 1/12 part, the English equivalent of the Latin uncia
(see def. [1] above). In medieval times, the word was used sometimes
for a unit of distance equal to 1/12 yard or
3 inches. It was also used for a unit of time
equal to 1/12 moment, or 7.5 seconds. In some
settings, an ounce of time was divided into exactly 47 atoms.
 ounce force (ozf or oz)
 a traditional unit of force, equal to the force experienced at the earth's
surface by a mass of one ounce. One ounce force equals 1/16 pound
force or about 0.278 014 newton.
 ounce mole (ozmol)
 a unit of amount of substance. One ounce mole of a chemical compound is
the same number of ounces as the molecular weight of a molecule of that compound
measured in atomic mass units. Thus the ounce
mole is equal to 28.349 52 moles.
 ounce per gallon (oz/gal)
 a traditional unit of mass concentration. One ounce per U.S. gallon equals
7.489 152 grams per liter (g/L). In Britain, 1 ounce per Imperial gallon is
equal to 6.236 023 grams per liter.
 ounce per square foot (oz/ft^{2})
 a traditional unit of density, still used widely in the U.S. for stating
the density of coatings, the "weight" of leather, the rates of application
for lawn chemicals, and many other applications. One ounce per square foot
is equal to 0.305 152 kilogram per square meter (kg/m^{2}).
 ounce weight (oz)
 a traditional unit for measuring the density (incorrectly called the "weight")
of a fabric. In most cases, the stated ounce density of a fabric is its density
in ounces per square yard (oz/yd^{2}). 1 ounce per square yard is
equal to 33.9057 grams per square meter (g/m^{2} or gsm). However,
when the fabric is shipped in rolls or bolts of a standard width, the ounce
density is sometimes figured in ounces per linear yard, the width being understood.
For example, for a bolt of wool having a standard width of 60 inches (1.524
meters), a density of 1 ounce per linear yard corresponds to 31.0034 grams
per linear meter, or, taking the width into account, 20.3434 grams per square
meter.
 oxgang
 an old English unit of land area equal to 1/8 hide
or roughly 15 acres (6 hectares).
The hide was considered the area a farmer could plow with a team of 8 oxen,
so an oxgang was the area he could plow with a single ox. The unit was also
called the bovate, from the Latin word bos for an ox or cow.
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