O

obol

a historic unit of weight or mass. The obol is a very small weight which originated as the weight of a tiny Greek coin. In 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 the Roman ounce (uncia) or about 0.57 gram.

o' clock [1]

a contraction of the phrase "of the clock," used in English after a statement of time. This phrase is a relatively recent invention; it has been traced to the early 1700's. 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 which works by describing an angle in terms of the face of a standard (12-hour) clock. Each hour "o' clock" spans an angle of 30°, so "4 o' clock" means an angle of 120° measured from dead ahead or some other agreed-upon point of reference.

octane number

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 iso-octane or, in the more precise language of chemical nomenclature, 2,2,4-trimethylpentane. (In an iso-octane 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 iso-octane 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 anti- knocking characteristics as the gasoline being tested. The octane rating is the percentage of iso-octane 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, and the number posted on the gas pump is the average of the iso-octane percentages obtained in the two methods.) There is a similar unit, cetane, used for rating diesel fuel.

octant

a unit of angle measure equal to 1/8 circle, 45°, or pi/4 radians. Or a unit of solid angle measure. One octant is 1/8 sphere, or pi/2 steradians, or about 5156.6 square degrees.

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.

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 a gallon. However, the gallon unit is missing in recent Spanish terminology. There are 4 octavillos in a cuartillo, 16 in an almude, and 48 in a fanega.

octet [1]

a unit of quantity equal to 8.

octet [2]

another name for a byte, used primarily in telecommunications. The name refers to the definition of a byte as 8 bits. Also, a group of 8 bits is called an octet, rather than a byte, in computers where the byte size is different from the usual 8.

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 an event has a-b (or a to b) odds in its favor, then the probability is being divided into a + b parts; the probability is a /(a + b) that the event will happen and b/(a + b) that it will not happen. In sports, however, the odds quoted are odds that a contestant will not win. Thus 4-1 odds on a horse in a horse race indicate that the probability of the horse's winning is 1/5. At these odds, betters pay $1 in order to win $4 if the horse wins. This is a gamble that the actual probability of the horse's finishing first is more than 1/5. 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 ampere-turns per meter. The unit honors Hans Christian Ĝrsted (1777-1851), 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. If a conductor is connected between two locations having different electric potential, then a current will flow through the conductor. The amount of the current depends on the potential difference, but it also depends on the extent to which the conductor resists the flow of current. This property of a circuit is called electric resistance, and it is measured in ohms. One ohm is the resistance which requires a potential difference of one volt per ampere of current. This is a small resistance; in practical circuits resistance is often measured in megohms, millions of ohms. The unit honors the German physicist Georg Simon Ohm (1787-1854). 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 was generally equal to 1/6 fuder or roughly 150 liters (40 U.S. gallons). The unit was often spelled aume in English.

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 cross-sectional area (in square meters) divided by its length (in meters).

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 the eastern Mediterranean. The oka is approximately 2.8 pounds or 1.28 kilograms; its size varied somewhat over the expanse of the area formerly part of the Turkish empire. The oka is also used sometimes as a unit of liquid volume equal to a little more than a liter, representing the volume (roughly 1.25 liters) occupied by an oka (weight) of water or wine.

old style (OS)

a notation used after a date to indicate that the date is stated in the Julian calendar (see year [2]). The Julian calendar was replaced by the Gregorian calendar in 1582 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 Britain, 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. See time

olympiad

a unit of time equal to four years. In ancient Greece, the olympiad referred to the four-year interval between successive Olympic Games. The first Greek olympiad was the period 776-773 BC. This usage was revived when the modern Olympics began, in 1896. The period 2001-2004, called the 27th Olympiad of the modern era, is also the 695th olympiad by the 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.

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 considered to be 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 10ⁿ times the smaller one.

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 which 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 which 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.

ounce (oz) [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 about 0.911 457 troy ounce. 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 Volume

ounce (oz) [2]

a second traditional unit of mass or weight. The troy ounce, traditionally used in pharmacy and jewelry, is 1/12 of a troy pound, or about 31.1035 grams. Thus the troy ounce equals 1.09714 avoirdupois ounces. This unit continues to be used as a conventional measure for gold and other precious metals; in particular, the prices of gold and silver (as quoted in financial markets) are the prices per troy ounce. The troy ounce is divided into 20 pennyweight or into 8 troy drams [2]. The troy ounce is sometimes abbreviated oz t to distinguish it from the more common avoirdupois ounce.

ounce (oz) [3]

a traditional unit of liquid volume, also called the fluid ounce (fl oz).

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 of 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 of force equals 1/16 pound of force or about 0.278014 newton.

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/yd²). 1 ounce per square yard is equal to 33.9057 grams per square meter (g/m² 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.