A

a
an international symbol for year, taken from the Latin word annus. Although English-speaking countries will certainly continue to use the traditional symbol yr for most purposes, scientists use the a symbol in papers and textbooks. The symbol is often seen in combinations such as Ma (million years) or Ga (billion years).
A
a symbol for international standard paper sizes, followed by the size number, as in A4 for a standard business-letter sheet.
A°
see angstrom star below.
aas
see as below.
ab-
a prefix indicating that an electrical unit is part of the CGS absolute electromagnetic system. These units are also indicated by the notation emu (as in "volt emu"). Although these units are defined naturally as part of the CGS system, all of them except the abampere and abcoulomb are either much too large or much too small for most applications. They have been replaced almost completely by the corresponding SI units. Following are the SI equivalents for each of the "ab" units:
Electric current: 1 abampere = 10 amperes
Electric charge: 1 abcoulomb = 10 coulombs
Capacitance: 1 abfarad = 109farads = 1 gigafarad
Inductance: 1 abhenry = 10-9 henry = 1 nanohenry
Resistance: 1 abohm = 10-9ohm = 1 nanohm
Conductance: 1 abmho = 109siemens
Magnetic flux density: 1 abtesla = 10-4tesla = 1 gauss
Potential: 1 abvolt = 10-8volt = 10 nanovolts
Power: 1 abwatt = 10-7watt = 0.1 microwatt
Magnetic flux: 1 abweber = 10-8weber = 1 maxwell.

acfm
abbreviation for "actual cubic feet per minute." See under sccm.
acoustic ohm
any one of several units measuring sound resistance. These units got their name by analogy with electric resistance, which is measured in ohms. The sound resistance across a surface in a given medium is defined to be the pressure of the sound wave at the surface divided by the "volume velocity," that is, the rate (volume per time) at which the medium is pushed perpendicular to the surface. Unfortunately, the result is often stated in "acoustic ohms" no matter what units are used to measure pressure and time. In the CGS system, the acoustic ohm equals 1 microbar second per cubic centimeter (µbar·s/cm3), which is the same as 1 dyn·s/cm5. In the MKS system, the acoustic ohm is equal to the SI unit, 1 pascal second per cubic meter (Pa·s/m3). The CGS acoustic ohm equals 105 MKS acoustic ohms. See also rayl.
acre (ac or A)
a unit of area used for measuring real estate in English-speaking countries. "Acre" is an Old English word meaning a field. Originally intended as the area that could be plowed in a day by a yoke of oxen, the acre was standardized, at least by the year 1200, to be the area of a field one furlong (40 rods or 10 chains) long by 4 rods (or 1 chain) wide. Thus an acre is 10 square chains, 160 square rods, 43 560 square feet or 4840 square yards. There are exactly 640 acres in a square mile. In metric countries the unit corresponding to the acre is the hectare, which is 10,000 square meters (the area of a square 100 meters on each side). One acre is equal to 0.404 687 3 hectare. See Area
acre foot (ac ft)
a unit of volume used to measure the capacity of reservoirs. One acre foot is a volume one foot deep covering an area of one acre. Thus an acre foot contains 43 560 cubic feet, or about 1233.482 cubic meters. See Volume.
acre inch (ac in)
a unit of volume, equal (of course) to 1/12 acre foot. An acre inch contains 3630 cubic feet, or about 102.7902 cubic meters.
AD
abbreviation for the Latin anno domini, "year of the Lord," the traditional designation for years of the common or Christian era. This abbreviation is often replaced by CE (common era), especially in countries where Christianity is not a dominant religion.
-ad
a suffix added to a number to create a unit of quantity equal to that number: for example, a 24ad is a unit of quantity equal to 24. Units of quantity equal to 1 through 8 are known, respectively, as the monad, dyad, triad, tetrad, pentad, hexad, heptad, and octad, terms coined by adding -ad to the Greek numbers 1-8.
Admiralty mile
See nautical mile.
aeon
a unit of time equal to one billion years (1 Ga). Proposed in 1957 for use in geology, the aeon is not approved by the SI and hasn't found much favor.
AFUE
an abbreviation for annual fuel utilization efficiency, a measure of the efficiency of a gas furnace. The rating is designed to represent the percentage of the fuel energy actually delivered as heat energy, averaged over the course of a typical heating season. The actual calculation is quite complex, taking many properties of the furnace into account. Older furnaces have ratings of 60% AFUE or even lower; the newest high-efficiency furnaces are rated in the 90%-95% AFUE range. The U.S. Department of Energy requires new furnaces to operate at 78% AFUE or better.
agate
a traditional unit of distance used in printing. The agate is usually considered equal to 1/14 inch (1.814 millimeters), since the traditional type size called agate set 14 lines to the inch (very small print!). In the more modern measuring system based on points [2], agate type has a height of 5.5 points; this would make the agate equal to about 0.076 inch or 1.933 millimeters.
air watt
an engineering unit used to express the effective cleaning power of a vacuum cleaner or central vacuum system. The air watt is practically the same as the ordinary watt. Measurements of vacuum power, however, are computed from English units using the following formula established by the American Society for Testing and Materials (ASTM): power in air watts equals 8.5·F·S, where F is the air flow in the system in cubic feet per minute (CFM) and S is the suction pressure in inches of water column (in WC). This definition makes the air watt equal to 0.9992 watt.
almude
a traditional unit of volume in Spain and Portugal. The two countries used the name for units of quite different sizes. The Spanish almude is comparable to the British gallon; it holds about 4.625 liters, equivalent to 1.017 imperial gallon or 1.222 U.S. liquid gallon. The Portuguese almude is much larger; it holds about 16.7 liters, which is 3.67 imperial gallons or 4.41 U.S. liquid gallons.
aln or alen
a traditional Scandinavian unit of distance very similar to the north German elle: roughly 2 feet or 60 centimeters. The Danish alen, also used in Norway, was equal to 62.77 centimeters (24.71 inches). The Swedish aln was 2 fot or 59.38 centimeters (23.38 inches).
alpha TE
an abbreviation for "alpha tocopherol equivalent," a measure of vitamin E used in nutrition. Vitamin E is actually a group of related chemical compounds called tocopherols. The activity, or potency, of vitamin E in a food or food supplement is measured by the quantity (in milligrams) of alpha tocopherol (the most active of the forms of the vitamin) which would be equivalent to the compounds present in the food or supplement. One milligram alpha TE is equal to 1.5 international units (IU).
alt h
traditional abbreviation in pharmacy for alternis horis, every other hour, a unit of frequency sometimes used in medical prescriptions.
AM or am
abbreviation for the Latin ante meridiem, "before noon," used after a time to indicate that the time is before 12:00 (noon).
amagat
units used by physicists to express the relative volume and density of gases. The amagat volume unit is about 22.414 liters per mole (L/mol) or 0.022 414 L/m3, the volume occupied by a gas at standard temperature (0.01°C) and standard pressure (1 atmosphere). The amagat density unit represents the corresponding relative density, which is equal to one kilomole per standard volume, or 44.615 moles per cubic meter (mol/m3), or 0.044 615 mole per liter (mol/L), again provided the measurement is made at standard temperature and pressure. In general, the ideal gas law shows that the relative density is equal to P/RT, where P is the pressure on the gas, T is the absolute temperature (in kelvins) and R is the universal gas constant, R = 8.314 joules per mole per kelvin. See Converter. The unit honors the Dutch physicist E. H. Amagat (1841-1915), whose work included the study of gases under pressure.
amber
an old English unit of volume, used for both liquids and dry goods. The amber was equal to about 4 bushels or roughly 140 liters.
American run
see run.
ampere (A or amp)
the fundamental SI unit of electric current, named for the French physicist André-Marie Ampère (1775-1836), one of the pioneers in studying electricity. The official definition of the ampere goes like this: suppose we have two parallel conductors, infinitely long and having negligible cross section. Place these conductors one meter apart in a perfect vacuum. One ampere is the current which, if it's flowing in these conductors, creates between them a force of 0.2 micronewtons per meter of length. (You're welcome to object that no one can make an infinitely long conductor, nor a perfect vacuum. But scientists can use the idealized definition to construct appropriate real-world equipment in their laboratories.) The other electrical units are all defined in terms of the ampere. For example, one ampere represents a current flow of one coulomb of charge per second. One ampere of current results from a potential distribution of one volt per ohm of resistance, or from a power production rate of one watt per volt of potential.
ampere hour (A·h or amp hr )
a commercial unit of electric charge, often used to state the capacity of a battery. One ampere hour is the charge accumulated by a steady flow of one ampere for one hour. This is equivalent to exactly 3600 coulombs.
ampere per meter (A/m) or ampere-turn per meter
the SI unit of magnetic field strength. One ampere per meter is equal to pi/250 oersteds (12.566 371 millioersteds) in CGS units. The ampere per meter is also the SI unit of "magnetization" in the sense of magnetic dipole moment per unit volume; in this context 1 A/m = 0.001 emu per cubic centimeter.
ampere turn (At)
the MKS unit of "magnetomotive force." Electric current passing through a coil of wire generates a magnetic field. This field-generating ability is called magnetomotive force; it is equal to the product of the current, in amperes, and the number of turns of wire in the coil. One ampere turn equals 4 pi/10 = 1.256 637 gilberts (Gb).
amphora
a historic unit of volume. An amphora is the volume of an urn or jar of the same name. These urns were tall, with handles near the top on both sides (the word amphora comes from two Greek words meaning "on both sides" and "carry"). Amphoras were the containers of choice for shipping wine and many other commodities in the ancient world. Archaeologists report that the Greek amphora held about 38.8 liters (10.25 U.S. liquid gallons, or 8.54 British imperial gallons). The Roman amphora was smaller, about 25.5 liters (6.74 U.S. gallons or 5.61 British imperial gallons).
angstrom (Å or A)
a metric unit of length, equal to 0.1 nanometer or 10-10 meter. Angstroms are used most often to measure the wave length of light waves. There is a technique called spectroscopy for identifying chemical substances by the wave lengths of light which they absorb (or emit, depending on the circumstances). Light shining through or from the substance is passed through a prism, which separates the various wave lengths to form a colorful spectrum. The spectrum often includes bright bands corresponding to wave lengths at which the substance emits light, or dark bands if the substance is absorbing light. The pattern of bands identifies the substance in somewhat the same way a fingerprint identifies a person. One of the pioneers of spectroscopy was the Swedish physicist Anders Jon Ångström (1814-1874), and in his honor the wave lengths of light waves are customarily stated in angstroms. Although English speakers usually pronounce the word as if it were English, the Swedish pronunciation is closer to "ong-strerm." See Length.
angstrom star (Å* or A*)
a unit used to measure the wavelength of X-rays. Because it's easier to measure the ratio between two X-ray wavelengths than it is to measure the wavelengths themselves, the wavelengths are usually stated as multiples of a standard wavelength. The X unit and the angstrom star are the units used for this purpose. Å* was defined by J.A. Bearden in 1965 to provide a unit approximately equal to the angstrom (10-10 meter or 0.1 nanometer). Later measurements have shown that in fact A* is equal to approximately 1.000 0015 x 10-10 meter or 100.000 15 picometers.
angular mil
see mil [2].
animal unit (AU)
a unit of feed consumption used in U.S. dairying and ranching. One animal unit is the feed or grazing requirement of a mature cow weighing 1000 pounds (453.59 kilograms). Total feed requirements are often figured by the animal unit month (AUM), the feed required to sustain one animal unit of livestock for one month.
anker
a small wine barrel used in Britain and elsewhere in northern Europe. In England an anker usually held 10 wine (U.S. liquid) gallons (37.85 liters); the Scottish anker held 20 Scots pints (about 34 liters). The word anker is of Dutch origin.
annual percentage rate (% APR)
a unit used in the U.S. for stating interest rates and rates of return on investment. By federal regulation, these rates can be stated however a financial institution wishes, but they must be stated also in % APR so that consumers can compare rates of different loans and investment opportunities. Mathematically, the natural rate r of return on money is the "instantaneous" rate, the rate that allows for compounding of interest continuously. The APR is the percentage growth rate a of the money over a period of one year, as if interest were compounded annually. The two rates are related by the formulas a = er - 1 and r = ln (1 + a), where er is the natural exponential function and ln is the natural logarithmic function.
Apgar score
a numerical measure of the health of a newborn baby. One minute after birth (and at regular intervals thereafter through the first moments of life) newborns are rated 0, 1, or 2 on five indicators of health (respiratory effort, heart rate, skin color, muscle tone, and reflexive response to smell). Possible scores therefore range from 0 to 10. The unit is named for its inventor, the American anesthetist Virginia Apgar (1909-1974).
apostilb (asb)
an MKS unit of luminance, representing the brightness of a surface uniformly radiating 1 lumen per square meter. This is the brightness produced by 1/picandela or 1 lux of light. Since the apostilb basically measures the same situation as the lux, it is rarely used.
arcminute (' or min)
a unit of angular measure equal to 60 arcseconds and to 1/60 degree. There are 21 600 arcminutes in a circle.
arcsecond (" or as or sec or s)
a unit of angular measure equal to 1/60 arcminute. One arcsecond is a very small angle indeed: there are 1 296 000 seconds in a circle. The SI defines s as the symbol for the time unit and recommends " as the symbol for the arcsecond. The symbol "as" has become common in astronomy, where very small angles are stated in milliarcseconds (mas).
are (a)
a basic unit of area in the metric system, equal to 100 square meters. The word is pronounced the same as "air." Being the area of a square 10 meters on each side, the are is a little large for measuring areas indoors and a little small for measuring areas outdoors. As a result, the are is not used as often as its multiple, the hectare (ha). One are is approximately 1076.3910 square feet, 119.6000 square yards, or 0.02471 acre.
arpent [1]
a traditional unit of distance in French-speaking countries. The arpent equals 30 toises or 10 perches; this is about 191.8 feet or 58.47 meters. The unit was used to measure land; in fact, arpentage is the French word for surveying. In Canada the arpent has an official definition of 191.835 English feet (58.471 308 meters).
arpent [2]
a traditional unit of area in French North America (Québec and Louisiana), equal to one square arpent [1]. The arpent of area equals 900 square toises, 100 (square) perches, approximately 0.8445 acre or 0.3419 hectare. By the official Canadian definition, the arpent of area contains 36 800.667 23 English square feet or about 0.844 827 acre (0.341 889 hectare).
arratel or artel
versions of a traditional Arab unit of weight; see rotl. The spelling arratel is used, for example, in Portugal, where the arratel is an alternate name for the libra.
arroba
a traditional unit of weight in Spain and Portugal, equal to 1/4 quintal. However, the Spanish and Portuguese quintals are of different sizes. In Spain, the arroba equals 25.36 pounds (11.50 kilograms); arrobas of very similar sizes were established in the Spanish speaking countries of Latin America. In Portugal and Brazil, the arroba equals 32.38 pounds (14.69 kilograms). The arroba has also been used as a metric unit equal to exactly 15 kilograms. The name of the unit comes from ar rub', Arabic for "the quarter."
arshin
a traditional Russian unit of distance. Peter the Great standardized the arshin at exactly 28 English inches, or 71.12 centimeters, early in the 1700s. The arshin was also used in several other countries adjacent to Russia. The arshin is also used as a unit of area equal to one square arshin; this would be equal to 5.4445 square feet or 0.5058 square meter.
artaba
a historic unit of volume, used for both liquid and dry measurement throughout the Middle East. In ancient times the artaba varied in size between about 35 and 55 liters; in recent centuries the Arab artaba, equal to about 66 liters, was a common unit in both Arab and non- Arab parts of the area.
as, ass, or aas
a traditional unit of mass for gold and silver, used in most of northern Europe but not in England. The unit varied in size from about 48 to 58 milligrams (0.75 to 0.90 English grain).
ASA number
for many years, the initials of the American Standards Association appeared on film packages in the United States as a measure of the speed of the photographic emulsion (the stuff on the film that "develops" to form the picture. The scale is arbitrary, but the important thing to know is that the speed at which the image registers on the film is proportional to the ASA number. Thus ASA 400 film registers an image twice as fast as ASA 200 film and four times as fast as ASA 100 film. The ASA number is now combined with the European DIN rating as a composite ISO rating. For example, ASA 400 film is now marked ISO 400/27°, because 27 is the DIN rating corresponding to ASA 400.
assay ton (AT)
a specialized unit of mass used by minerologists in assaying (testing) ores for the presence of gold, silver, platinum, uranium, or other valuable metals. One assay ton equals 29.167 grams: just a little over an ounce! The assay ton is actually a sample size. Since there are 29 167 troy ounces in a short ton (2000 lb), the number of milligrams of a precious metal in a sample of one assay ton is numerically equal to the number of troy ounces of that metal in one ton of raw ore. In Britain, the assay ton is based on the long ton and thus equals 32.667 grams.
astronomical unit (ua or au)
a unit of distance used by astronomers to measure distances in the Solar System. One astronomical unit equals the average distance from the center of the Earth to the center of the Sun. The currently accepted value, adopted in 1996, is 149 597 870 691 meters (1.495 978 706 91 x 108 kilometers or about 92 955 807 miles), with an uncertainty of about 30 meters. The astronomical unit is a convenient yardstick for measuring the distances between objects in the Solar System. Astronomers find it particularly convenient to use astronomical units in solving the equations of planetary motion. Because these equations are the same regardless of the unit used for distance, the predictions they generate will remain correct even if future astronomers determine a slightly different length for the distance between the Earth and Sun. The official symbol for the unit is ua, but the symbol au is also used. This unit is accepted for use with SI units. See Length.
atmo- meter (atmo-m)
a unit used in atmospheric physics to compare the "depth" or total volume of atmospheres, or components of atmospheres. The depth (in atmo-meters) is equal to the depth (in meters) the atmosphere, or one gas component of the atmosphere, would have if it formed a uniform layer at standard temperature (0 °C) and pressure (1 atmosphere). One atmo-meter represents 2.686 99 x 1025 molecules of gas per square meter of planetary surface.
at
symbol for the technical atmosphere.
atmosphere (atm or atmos)
a unit of pressure designed to equal the average pressure of the Earth's atmosphere at sea level. In other pressure units, one atmosphere equals exactly 1013.25 millibars (mb), 101.325 kilopascals (kPa), approximately 29.92 inches of mercury (in Hg), or 14.6959 pounds of force per square inch (lb/in2). See Converter. This is the standard atmosphere; it equals 1.0332 technical atmosphere.
atomic mass unit (u or amu)
the unit of mass used by chemists and physicists for measuring the masses of atoms and molecules. Early in the nineteenth century, scientists discovered that each chemical element is composed of atoms, and that each chemical compound is composed of molecules in which atoms are combined in a fixed way. No one knew then just how small atoms and molecules really are, but as long as the relative weights of the different atoms were known, the outcome of chemical reactions could be predicted. These relative masses were determined by careful study of various reactions. The general idea was that atoms of hydrogen, known to be the lightest element, should have a mass of 1 amu, and all the other atoms should have masses which are whole-number multiples of this (then unknown) mass of the hydrogen atom. For a long time, physicists and chemists disagreed on the details of this definition. In 1960 they agreed on the definition of the unified atomic mass unit as 1/12 the mass of the most common atoms of carbon, known as carbon-12 atoms. (Most elements are mixtures of atoms which have different masses because they contain different numbers of neutrons; these varieties are called isotopes.) Careful experiments have measured the size of this unit; the currently accepted value (1998) is 1.660 538 73 x 10-24 grams. (This number equals 1 divided by Avogadro's number; see mole.) In addition, 1 amu equals approximately 931.494 MeV (see electron volt).
atomic number
The atomic number was originally defined (about 1865) simply as an index describing the position of an element in the periodic table. Not until 1913 was it known that the atomic number is actually a unit of measurement, equal to the number of electrons surrounding a neutral (uncharged) atom, and also to the number of protons in the nucleus.
atto- (a-)
a metric prefix denoting 10-18 (one quintillionth). For example, one electron volt equals 0.1602 attojoule. The root of the prefix is atten, the word for 18 in Danish and Norwegian. See Prefix
AUC
abbreviation for the Latin phrase ab urbe condita, "from the founding of the city." The Romans counted years from the legendary founding of Rome by Romulus and Remus, an event placed in 753 BC (or BCE) in our present calendar. The Christian Church continued to count years AUC for centuries after the fall of Rome. Our year 2000 is 2753 AUC.
aught (/0)
a unit used by jewelers and craftspeople to measure the size of small beads (often called seed beads). The measurement scale is inverted: larger numbers of aughts correspond to smaller beads. Beads of size 11/0, a common size, average a little less than 2 millimeters in diameter, and other sizes are more-or-less inversely proportional. The measure may have originated as the number of beads that could comfortably be strung on one inch of cord; with present sizes a string of n beads of size n/0 occupies about 0.8 inch (20 mm). The word aught, meaning zero, is a fairly recent corruption of the old English word naught, meaning nothing; apparently the phrase a naught, meaning a zero, came to be misspelled as an aught.
aume
an old English wine measure equal to about 40 gallons (roughly 150 liters). The aume is the English version of a German unit, the ohm [2].
aune
a traditional French unit of distance, varying from region to region but similar in length to the English.