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a unit of area used for measuring real estate.  The acre was originally defined as the area that could be plowed in a day by a yoke of oxen. It was in use in England at least as early as the eighth century, and by the end of the ninth century it was generally understood to be the area of a field one furlong(40 rods or 10 chains) long by 4 rods (or 1 chain) wide. an acre is 10 square chains, 160 square rods.  See Unit Table or Conversion Caculator.
acre foot
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 exactly 43 560 cubic feet, about 325 851.4 U.S. gallons, or about 1233.482 cubic meters (0.123 348 hectare meter).
acre inch
a unit of volume equal (of course) to 1/12 acre foot. An acre inch contains exactly 3630 cubic feet, about 27,154.29 U.S. gallons, or about 102.7902 cubic meters.
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.
air mass
a unit used in astronomy in measuring the absorption of light from the stars by the atmosphere. One air mass is the amount of absorption of light from a star directly overhead (at the zenith). The absorption of light from other stars is greater, because their light must pass obliquely through the atmosphere.
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.
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). The notations "AM" and "PM" are used extensively in the United States, where time is usually not stated on a 24-hour basis. Note: the ambiguous notation "12:00 am" should not be used, since it may be taken to mean either noon or midnight (the most common usage today is that 12:00 am is midnight).
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.
ampere (A or amp)
the SI base unit of electric current, named for the French physicist André-Marie Ampère (1775-1836), one of the pioneers in studying electricity. 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. The unit is known informally as the amp, but A is its official symbol.
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, name for Swedish physicist Anders Jon Ångström (1814-1874).
angstrom star (Å* or A*)
a unit used to measure the wavelength of X-rays.  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.
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). This is approximately 26 pounds (11.8 kg) of dry forage. 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 (780 pounds or 354 kg).
anker or anchor
a small wine barrel used in Britain and 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  a financial institution wishes, but they must be stated also in % APR so that consumers can compare rates of different loans and investment opportunities.  The APR is the percentage growth rate a of the money over a period of one year, as if interest were compounded annually.
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).
apothecary weights
a version of the troy weight system formerly used by apothecaries (pharmacists). The troy pound, equal to 373.242 grams or 13.165 72 avoirdupois ounces, is always divided into 12 ounces, each ounce being equal to 480 grains. In the apothecary system, the ounce is divided into 8 drams (60 grains) each containing 3 scruples (20 grains). There is a parallel system of liquid volume measure in which the fluid ounce is divided into 8 fluid drams (or fluidrams) each containing 3 fluid scruples. The apothecary system continued in use until the early 20 century, but it has been replaced  in pharmacy by the use of metric units.
arcminute (' or min)
a unit of angular measure, also called the minute of arc, 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, also called the second of arc, equal to 1/60 arcminute. One arcsecond is a very small angle indeed: there are 1 296 000 seconds in a circle. The SIdefines 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).
a unit of area 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, 119.5990 square yards, or 0.02471 acre.  See Unit Table.
in English the term "arm's-length" is used figuratively to mean "a distance discouraging familiarity or conflict." In many cultures, however, the length of a human arm is standardized as a unit of distance equal to about 70 centimeters or 28 inches. Examples include the Italian braccio, the Russian sadzhen, and the Turkish pik.
a traditional Russian unit of distance.  The standardized arshin is exactly 28 English inches, or 71.12 centimeters,  in the early 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.
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.
astronomical unit (ua or au 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 (it is the length of the semimajor axis of the Earth's elliptical orbit, which is the ordinary average of the Earth's minimum and maximum distances from the Sun).  The astronomical unit is a convenient yardstick for measuring the distances between objects in the Solar System.  This unit is accepted for use with SI units. The official symbol for the unit is ua, but the symbol au is common in English-speaking countries. 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.
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), 760.0 millimeters of mercury (mm Hg), or 14.6959 pounds of force per square inch (lbf/in2). This is the standard atmosphere; it equals 1.0332 technical atmosphere.
a medieval unit of time. In medieval times, the Latin form atomus was  used to mean "a twinkling of the eye,"  the smallest amount of time imaginable. This was sometimes defined in a precise way equivalent to exactly 1/376 minute or about 160 milliseconds.
atomic mass unit
the unit of mass used by chemists and physicists for measuring the masses of atoms and molecules.  No one knew 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. In 1960, scientists 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. 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.)
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 t 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.
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.


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