G

g: a symbol for the average acceleration produced by gravity at the Earth's surface (sea level). The actual acceleration of gravity varies from place to place, depending on latitude, altitude, and local geology. The symbol g is often used informally as a unit of acceleration. By agreement among physicists, 1 g is defined to be exactly 9.806 65 meters per second per second, or about 32.17405 feet per second per second, a value known as standard gravity. At latitude p, a conventional value of the acceleration of gravity at sea level is given by the International Gravity Formula,; g = 978.0495 [1 + 0.0052892 sin²(p) - 0.0000073 sin² (2p)] centimeters per second per second.; The symbol g was used as a unit first in aeronautical and space engineering, where it is important to limit the accelerations experienced by the crew members of aircraft and spaceships: the "g forces," as they are called. This use became familiar through the space programs, and now a variety of accelerations are measured in g's. The name grav is also used for this unit. Note that g is also the symbol for the gram. See Accleration.
G: informal abbreviation in computer science for 2³⁰ = 1,073,741,824. See also giga- [2].
Ga: symbol for one billion (10⁹) years. The a stands for the Latin annus, year.
gage: alternate spelling for gauge (see below).
galactic year: the unit of time in which the Solar System makes one revolution around the Milky Way galaxy. The galactic year is estimated to be about 225 million ordinary years. The age of the Solar System is about 20 galactic years.
galileo (Gal or gal): the CGS unit of acceleration. One galileo is an acceleration of 1 centimeter per second per second. This unit is used by geologists, who make careful measurements of local variations in the acceleration of gravity in order to draw conclusions about the geologic structures underlying an area. These variations are typically measured in milligals (mGal). One Gal is approximately 0.0010197 g, so a milligal is a very small acceleration, about 10^-6 g. The name of the unit honors the Italian astronomer and natural philosopher Galileo Galilei (1564-1642), who proved that all objects at the Earth's surface experience the same gravitational acceleration. To avoid confusion with the symbol for the gallon, and to conform to the usual metric style, the symbol for this unit should be Gal rather than gal. See Accleration.
gallon (gal) [1]: a traditional unit of liquid volume, derived from the Roman galeta, which originally meant a pailful. Gallons of various sizes have been used in Europe ever since Roman times. In the United States, the liquid gallon is legally defined as exactly 231 cubic inches; this is equal to the old English wine gallon, standardized during the reign of Queen Anne in 1707. The U. S. gallon holds 4 liquid quarts or exactly 3.785 411 784 liters; a U.S. gallon of water weighs about 8.33 pounds. American colonists were also familiar with an older unit, the Elizabethan beer and ale gallon, which held 282 cubic inches (4.621 liters). See Volume.
gallon (gal) [2]: a traditional unit of dry volume. The U. S. dry gallon is 1/2 peck, or 4 dry quarts, or 268.8025 cubic inches, or approximately 4.404 884 liters. This unit is the English corn or grain gallon, standardized during the reign of Elizabeth I in the sixteenth century. The earliest official definition of a dry gallon in Britain is a 1303 proclamation of Edward I, where the gallon is defined as the volume of 8 pounds of wheat; the current U.S. gallon contains about 7.5 pounds of wheat. See Volume.
gallon imperial (gal) [3]: currently the British use a larger gallon than either of the American gallons. The Imperial Weights and Measures Act of 1824 established a new unit for all volumes, liquid or dry, replacing all the other gallons in previous use in Britain. The imperial gallon, designed to contain exactly 10 pounds of distilled water under precisely defined conditions, holds exactly 4.546 09 liters or approximately 277.4194 cubic inches. The imperial gallon equals 1.20095 U.S. liquid gallons or 1.03206 U.S. dry gallon. See Volume.
gamma [1]: a unit of magnetic flux density equal to 10^-9tesla (1 nanotesla) or 10^-5oersted (10 µOe). In geophysics, small changes in the Earth's magnetic field are traditionally stated in gammas. The nanotesla (nT) is now recommended for these measurements.
gamma [2]: an informal metric unit of mass equal to 1 microgram (µg).
garnets: a traditional unit of liquid volume in Russia. The garnets is equivalent to approximately 3.28 liters; this is about 3.47 U.S. quarts or 2.89 British imperial quarts. See Volume.
gauge (ga) [1]: a traditional unit measuring the interior diameter of a shotgun barrel. The gauge of a shotgun was the number of lead balls, each of a size just fitting inside the barrel, which were required to make up a pound. In other words, if a lead ball weighing 1/12 pound just fit in the barrel of a shotgun, then it was a 12-gauge shotgun.
gauge (ga) [2]: a unit expressing the fineness of a knitted fabric, equal to the number of loops per 1.5 inches (38.1 millimeters). The same unit is also used to express the size of the knitting needles used to create a fabric of that fineness.
gauge (ga) [3]: a traditional unit measuring the diameter (or the cross-sectional area) of a wire. Various wire gauge scales have been used in the U.S. and Britain. In traditional scales, larger gauge numbers represent thinner wires. In the American Wire Gauge (AWG) scale, 0000 gauge represents a wire having a diameter of 0.46 inch and 36 gauge represents a diameter of 0.005 inch (5 mils). Diameters for the other gauges are obtained by geometric interpolation, meaning that the ratio between successive diameters is a constant, except for necessary roundoff. Thus n gauge wire has a diameter of .005·92^((36-n)/39) inch (gauges 00, 000, and 0000 are interpreted as -1, -2, and -3, respectively, in this calculation). The metric wire gauge number is equal to 10 times the diameter of the wire, in millimeters; thus a metric 8 gauge wire has diameter 0.8 millimeters.
gauge (ga) [4]: a traditional unit measuring the thickness of sheet metal. Larger gauge numbers represent thinner metal: 10 gauge represents a thickness of 0.1345 inch (3.416 millimeters) and each increase of 1 in the gauge number corresponds to a reduction of about 10% in the thickness.
gauss (G or Gs) [1]: the CGS unit of magnetic flux density. A field of one gauss exerts, on a current-carrying conductor placed in the field, a force of 0.1 dyne per ampere of current per centimeter of conductor. One gauss represents a magnetic flux of one maxwell per square centimeter of cross-section perpendicular to the field. In SI units, one gauss equals 10^-4 tesla. The unit is named for the German mathematician and astronomer Karl Friedrich Gauss (1777-1855).
gauss (G or Gs) [2]: a former name for the CGS unit of magnetic field strength, officially renamed the oersted in 1930.
gauss (G or Gs) [3]: the CGS unit of magnetic dipole moment per unit volume, more commonly written emu/cm³ or emu/cc. In this use the gauss equals 1000 amperes per meter in SI units.
GDU: a symbol for gelatin digesting unit, used for measuring dosage of bromelain, an enzyme used as a digestive aid and for reduction of pain and inflammation. This unit cannot be converted to a weight unit, because different preparations of the enzyme differ in activity. Bromelain is also measured in milk clotting units (MCU); 1 GDU equals approximately 1.5 MCU.
gee pound: another name for a slug. See Mass.
generation (gen): an informal unit of time. Roughly speaking, a generation is the average length of time between the birth of a parent and the birth of the child. This leaves a question, however: should just the father, or just the mother, or both parents be included in the calculation? Various answers to this question, plus a lack of consistent data, have led to a range of estimates for the length of a generation, from about 25 to 35 years. Genealogists tend to use the higher figures, anthropologists the lower ones. There is some research suggesting that the approximate length of the generation in the world today is about 28 years.
geographical mile: another name for the nautical mile, especially the Admiralty mile (6080 feet or 1853.184 meters). For a (different) German use of the term geographische meile, see meile.
GeV: the symbol for one billion (10⁹) electronvolts. Thanks to Einstein's equation E = mc² equating mass wth energy, the GeV can be regarded either as a unit of energy equal to 160.217 646 2 pico joules, or as a unit of mass equal to 1.782 662 x 10^-24 gram or 1.073 544 atomic mass unit.
g/g: a symbol for "gram per gram", a unit of mass concentration. For example, a concentration of 0.02 g/g, or 2% g/g, means that the substance being measured comprises 2% of the mass of the mixture in which it is found. This symbol is equivalent to the traditional symbol w/w.
gibi- (Gi-): a binary prefix meaning 2³⁰ = 1 073 741 824. This prefix, adopted by the International Electrotechnical Commission in 1998, replaces giga- for binary applications in computer science. The prefix is a contraction of "gigabinary."
gig: an informal contraction of the gigabyte (GB) common in computer science, especially as a unit of storage capacity.
giga- (G-): a metric prefix denoting 10⁹ or one billion (in the American meaning of the word billion). This prefix was coined from the Greek word gigas, meaning giant. The Greeks pronounced the word with a hard g, as in "gig." However, the prefix is usually pronounced with a soft g, as in "jig." Dictionaries usually list the hard g pronunciation as a second choice. See Prefix
gigabecquerel (GBq): a unit of radioactivity equal to one billion atomic disintegrations per second or 27.027 milli curies.
gigaflops (Gflops): a unit of computing power equal to one billion (10⁹) floating point operations per second. See flops.
gigajoule (GJ): a metric unit of energy commonly used in the energy industry. One gigajoule equals 947 817 Btu, 277.7778 kilowatt hours, or about 9.48 therms.
gigameter (Gm): a metric unit of distance equal to one million kilometers or about 621 371.2 miles. Seldom used, this unit would be a good yardstick for the inner Solar System: the distance from the earth to the sun is about 149.60 gigameters.
gigan: One Billion Years. See Time Increments
gigapascal (GPa): a metric unit of pressure. One gigapascal equals 10 kilobars or approximately 145 038 pounds (72.519 short tons) per square inch. Pressures in this range are common inside the earth and can be produced by various high-energy events.
gigawatt (GW): a metric unit of power equal to one billion (10⁹) watts or about 1.341 million horsepower.
gigawatt hour (GW·h): a metric unit of energy equal to one billion kilowatt hours, 3.6 terajoules, or about 3.412 billion Btu.
gilbert (Gi): the CGS unit of magnetomotive force, equal to 10/4 pi = 0.795 775 ampere-turns. The unit is named for the English physicist and physician William Gilbert (1540-1603).
gill (gi): a traditional unit of volume for liquids, especially wine and other alcoholic beverages. The gill is 1/4 pint. In the U. S. customary system, one gill is equal to 1/2 cup, 4 fluid ounces, 7.21875 cubic inches, or about 118.3 milliliters. In the British Imperial system, the gill equals 5 fluid ounces, 8.670 cubic inches, or about 137.7 milliliters. The unit is pronounced "jill", with a soft "g" sound. See Volume
gillion: an informal alternate name for the number 10⁹, called "billion" in America but often called "milliard" in France and "thousand million" in Britain. The reasoning here is that if mega- means a million and tera- a trillion, then giga- should mean a gillion!
gin: one of several spellings used in English for the jin, the Chinese weight unit also called the catty.
g/kg: a symbol for "gram per kilogram", a unit of mass concentration equal to 1 per mill (1 part per thousand). 1 g/kg also equals 0.1% g/g or 0.1% w/w.
glass [1]: a unit of time measured by an hourglass or sandglass. At sea, time was traditionally measured with half- hourglasses, making the glass a nautical unit of time equal to 1/2 hour. In this use the glass is another name for the bell.
glass [2]: another name for the U.S. cup (236.6 milliliters). Doctors in the U.S. are fond of saying that everyone should drink 8 glasses of water a day, and this is the amount they have in mind for a glass.
glass [3]: an informal unit of volume used in Australian pubs. In the eastern states of Australia a glass of beer is usually 200 milliliters, but in Western Australia it is 285 milliliters.
glean: an old English unit of quantity for herrings, equal to 25 fish.
gm: an incorrect symbol for the gram (see below). The correct symbol is g.
GMT: abbreviation for Greenwich Mean Time, the standard time of longitude 0°. This meridian of longitude, called the prime meridian, was fixed as the longitude of the Royal Greenwich Observatory in London by the International Meridian Conference of 1884. GMT is five hours later than U.S. Eastern Standard Time. The abbreviation UT (Universal Time) has largely replaced GMT.
go: a traditional Japanese unit of liquid volume. One go is about 180.39 milliliters, 0.3812 U.S. pint, or 0.3174 British imperial pint.
goad: a traditional unit of distance sometimes used in measuring cloth. One goad is equal to 54 inches or 1.5 yards (1.3716 meters). A goad was originally a spear; later it was a pointed rod used for prodding animals to get a move on. The unit must have originated as the length of such an instrument.
gon (^g): another name for the grad, a unit of angle measurement equal to 1/400 circle, 0.01 right angle, 0.9°, or 54'. This name is used especially in German, Swedish, and other languages in which the word grad means degree. The name is taken from the ancient Greek word gonia, angle.
googol: a unit of quantity equal to 10¹⁰⁰ (1 followed by 100 zeroes). The googol was invented by the American mathematician Edward Kasner (1878-1955) in 1938. According to the story, Kasner asked his nephew Milton Sirotta, who was then 8 years old, what name he would give to a really large number, and "googol" was Milton's response. Kasner also defined the googolplex, equal to 10^googol, that is, 1 followed by a googol of zeroes. These inventions caught the public's fancy and are often mentioned in discussions of very large numbers. In the traditional American system for naming large numbers, the googol is equal to 10 dotrigintillion.
gpg: customary symbol for grains per gallon (gr/gal), a traditional unit measuring the hardness of water. Water is "hard" if it contains dissolved minerals such as calcium or magnesium salts. 1 gpg is equivalent to about 17.118 milligrams per liter (mg/L). This unit is also called the Clark degree; see degree [4].
gpm, gps: customary symbols for gallons per minute (gal/min) and gallons per second (gal/s), traditional units for measuring the flow of liquids. 1 gpm equals about 3.785 41 liters per minute (L/min) if U.S. gallons are meant, or exactly 4.546 09 L/min if British imperial gallons are meant.
gr: an ambiguous symbol, traditionally used in English both for the grain and the gross. It has also been used, very improperly, for the gram (g).
grad or grade or gon (^gorgrd) [1]: a unit of angle measurement equal to 1/400 circle, 0.01 right angle, 0.9°, or 54'. This unit was introduced in France, where it is called the grade, in the early years of the metric system. The grad is the English version, apparently introduced by engineers around 1900. The name gon is used for this unit in German, Swedish, and other northern European languages in which the word grad means degree. Although many calculators will display angle measurements in grads as well as degrees or radians, it is difficult to find actual applications of the grad today.
grade [2]: a measure of the steepness of a slope, such as the slope of a road or a ramp. Usually stated as a percentage, the grade is the same quantity known as the slope in mathematics: the amount of (vertical) change in elevation per unit distance horizontally ("rise over run"). Thus a 5% grade has an elevation gain of 0.05 meter for each meter of horizontal distance, or 0.05 foot for each foot of horizontal distance. The angle of inclination, in grads or grades [1], is not equal to the percentage grade in this sense: for a 5% grade the angle of inclination is about 2.86° or 3.18 grads.
grade [3]: a measure of quality for ball bearings. A bearing of grade g is required to be spherical to an accuracy of g parts per million (g 10^-6). Thus lower grade numbers represent better bearings; a 25 grade bearing is spherical to within 25 ppm, but a 1000 grade bearing is only spherical to within 1000 ppm, or 0.1%.
grade point (gp): a unit of academic recognition used in U.S. schools and colleges. To compute the number of grade points awarded to a student for a course, the student's grade (often assigned as a letter or a percentage) is converted to a standard scale (traditionally 0.0-4.0, with 4.0 being the highest score). This scale number is multiplied by the number of semester hours or quarter hours assigned to the course. Thus a student who has a grade of A (4.0) on a course carrying 3 semester hours of credit receives 12 grade points.
grain (gr) [1]: a traditional unit of weight. The grain, equal to 1/480 troy ounce (see also pound [2]), or exactly 64.798 91 milligrams, was the legal foundation of traditional English weight systems, with various pounds being defined as a specified number of grains: 5760 grains in a troy pound and 7000 grains in an avoirdupois pound, for example. In the version of the troy system used by jewelers, there are 24 grains in a pennyweight and 20 pennyweight in an ounce. In the version used by apothecaries, there are 20 grains in a scruple, 3 scruples in a dram, and 8 drams in an ounce.
grain (gr) [2]: a unit of weight formerly used by jewelers in measuring diamonds and other precious stones. The jeweler's grain is exactly 1/4 carat. Now that the carat has been standardized at 200 milligrams, the jeweler's grain is exactly 50 milligrams, or approximately 0.7716 troy grain.
grain (gr) [3]: a traditional French unit of weight equal to 53.115 milligrams.
grain per gallon (gr/gal or gpg): see gpg above.
gram (g): the basic unit of mass in the metric system. The name comes the Latin gramma, which was a small weight similar to the English grain. The gram was originally defined to be the mass of one cubic centimeter of pure water, but to provide precise standards it was necessary to construct physical objects of specified mass. One gram is now defined to be 1/1000 of the mass of the standard kilogram, a platinum-iridium bar carefully guarded by the International Bureau of Weights and Measures in Paris for more than a century. The gram is a small mass, equal to about 15.432 grains or 0.035 273 966 ounce. The original French spelling gramme is sometimes used. Note: The only correct symbol for the gram is g. The abbreviations "gm" and (worse) "gr" should never be used.
gram atom, gram atomic weight: older names for the atomic mass unit.
gram calorie: the CGS unit of heat energy; see calorie.
gram equivalent (gEq), gram equivalent mass (Gem), gram equivalent weight (GEW): various names for the mass in grams of a substance that would react with or replace one gram of hydrogen. See equivalent, the shorter name now used in many contexts for this unit.
gram mole, gram molecule, gram molecular weight (gmol or gmole): older names for the mole.
gram rad: a name sometimes used for the rad, a unit of radiation dose.
gram weight or gram force (gf): an informal unit of weight equal to the force exerted on a mass of one gram by gravity at the Earth's surface: approximately 980.665 dynes, 9.806 65 millinewtons, or about 0.002 2045 pounds of force in the English system.
gran: a traditional German weight unit, varying in size but typically about 60 milligrams.
grano: a traditional Italian weight unit, varying in size but typically about 50 milligrams. The plural is grani.
grav (g): another name for the unit of acceleration usually called the g (see above).
gray (Gy): the SI unit of radiation dose. Radiation carries energy, and when it is absorbed by matter the matter receives this energy. The dose is the amount of energy deposited per unit of mass. One gray is defined to be the dose of one joule of energy absorbed per kilogram of matter, or 100 rad. The unit is named for the British physician L. H. Gray (1905-1965), an authority on the use of radiation in the treatment of cancer.
great gross: a traditional unit of quantity equal to a dozen gross, or 1728.
great hundred: a traditional unit of quantity equal to 120 instead of 100. The great hundred equals 6 score or 2 shocks. Merchants in medieval England used both the great hundred and the gross in specifying quantities, but more recently the gross has been much more common.
Gregorian year: a unit of time equal to exactly 365.2425 days, the average length of a year in the Gregorian calendar currently used throughout the world. See year [2].
grit: a measure of fineness for abrasive materials such as sandpaper, sanding belts, or the finer materials used to polish optical surfaces. Originally the grit number was the number of holes in a standard screen; if the screen had, say, 240 holes, then the particles that would pass through the screen were described as 240 grit. However, very fine abrasive particles (such as 500 to 1000 grit) are too small to be screened in this way, so the measure is defined by tables giving the average particle size in micrometers for each grit size.
gros: a traditional French weight unit equal to 3 deniers (about 3.824 grams).
gross (gro or gr): a unit of quantity equal to a dozen dozen, or 144. This commercial unit has been in use since at least the 1400's.
gross ton (GT): the name "gross ton" is used in at least two ways: (1) as another name for the British Imperial or long ton of 2240 pounds (see ton [1]), and (2) as another name for the register ton, a unit of volume equal to 100 cubic feet (see ton [3]), in describing the entire interior volume of a ship as opposed to the cargo-carrying capacity. To avoid confusion, it is better to use "long ton" for use (1) and "gross register ton" for use (2).
gry: a proposed unit of distance in the English traditional system. The name was first used in June 1679 by the philosopher John Locke (1632-1704) as a unit equal to 0.001 foot, 0.01 inch, or 0.1 line in a decimalized distance system. (Thomas Jefferson, who was very familiar with Locke's writings, later proposed a similar system in the U.S., but he called 0.001 foot a point rather than a gry.) In 1813, the gry was revived in another decimal measurement scheme in Britain. All these ideas failed, but the gry had some limited use in the nineteenth century as a unit equal to 0.1 line or 1/120 inch (0.211 667 millimeter). Long forgotten, the gry has recently come back into the limelight in connection with a puzzle, circulating on the Internet, which asks for three English words ending in -gry. The word "gry" is from the ancient Greek, where it meant "a trifling amount."
gsm: a common but non-standard symbol for grams per square meter (g/m²), the metric unit of density for paper and for fabric. Paper density measured in this unit is often called grammage.
gt, gtt: traditional pharmicist's abbreviations for a drop. Originally, gt was the singular (1 gt) and gtt the plural.
g/t: symbol for grams per tonne (metric ton), a unit of proportion equal to 0.001 g/kg or 1 part per million by mass.
Gunter's chain: the traditional surveyor's chain equal to 4 rods; see chain.