M

M [1]
informal abbreviation for million in expressions where the base unit is understood, as in "500M hard drive" (500 megabytes or mebibytes). In chemistry, M is the symbol for "molar" (see below).
M [2]
the Roman numeral 1000, sometimes used in symbols to indicate a thousand, as in Mcf, a traditional symbol for 1000 cubic feet. This use of the letter M is very confusing and should be scrapped.
Ma
a symbol for one million years, often used in astronomy and geology.
mace
a traditional Chinese unit for weighing precious metals, especially silver. In the colonial period, the mace was considered to equal 0.1 tael or liang; this would be 2/15 ounce or about 3.78 grams.
mach (M or Ma)
a measure of relative velocity, used to express the speed of an aircraft relative to the speed of sound. The name of the unit is placed before the measurement. Thus "mach 1.0" is the speed of sound, "mach 2.0" is twice the speed of sound, and so on. The actual speed of sound varies, depending on the density and temperature of the atmosphere. (At 0°C and a pressure of 1 atmosphere the speed of sound is about 1088 ft/sec, 331.6 m/sec, or 741.8 mi/hr). The mach speed is important to the control of an aircraft, especially at speeds close to or exceeding mach 1.0. The unit, usually pronounced "mock" in English, is named for the Austrian physicist Ernst Mach (1838-1916). See Speed
magnitude (mag) [1]
a unit traditionally used in astronomy to express the apparent brightness of stars, planets, and other objects in the sky. For centuries, the brightest stars were said to be of the "first magnitude," with fainter ones of the "second magnitude" and so on down to "sixth magnitude" for the faintest stars visible to the unaided eye. When it became possible to measure stellar brightnesses precisely, it was discovered that stars of a given traditional magnitude were roughly 2.5 times brighter than stars of the next magnitude. Astronomers agreed to define the magnitude scale so that a difference of exactly 5.0 mag corresponds to a brightness difference of exactly 100 times. A difference of 1.0 mag then corresponds to a brightness difference of the fifth root of 100 or about 2.512 times. The scale is upside down: brighter stars have lower, not higher magnitudes, in keeping with the historical origin of the scale. The zero point (0.0 mag) is set arbitrarily so that the stars historically listed as "first magnitude" have magnitude measurements of 1.5 mag or brighter.
magnitude (mag) [2]
a unit used in earth science to measure the intensity of earthquakes. Geologists actually use several scales to measure earthquake intensity, but the one best known to the public is the Richter magnitude scale, developed in 1935 by Charles F. Richter (1900-1985) of the California Insitute of Technology. The Richter magnitude is computed from the measured amplitude and frequency of the earthquake's shock waves received by a seismograph, adjusted to account for the distance between the observing station and the epicenter of the earthquake. An increase of 1.0 in the Richter magnitude corresponds to an increase of 10 times in the amplitude of the waves. The most powerful earthquakes recorded so far had magnitudes of about 8.5. The Richter magnitude measures the intensity of the earthquake itself, not the intensity of the earthquake's effects: the effects also depend on the depth of the earthquake, the geology of the area around the epicenter, and many other factors.
magnum
a traditional unit of volume for wine, generally equal to 2 bottles. This is now exactly 1.5 liters (about 2.114 U.S. quarts).
mahnd
a traditional Arab weight unit equal to about 2.04 pounds or 907 grams.
mål
a Norwegian word for "measure," mål has been used as a name for various traditional Norwegian units. As a land measure, the mål is currently defined to be the same as the dekare, that is, exactly 1000 square meters (0.1 hectare or 0.2471 acre). The mål has also been used as a unit of volume equal to the dekaliter (10 liters).
mandel
a traditional German unit of quantity equal to 15.
man hour
a common unit of labor equal to the work of one person for one hour. The less restrictive term person hour is gradually coming into use.
manpower
an informal unit of power equal to 0.1 horsepower or about 74.57 watts. The unit seems to have been invented by American engineers.
manzana
a traditional unit of land area in Central America. The manzana is the area of a square 100 varas on a side; it thus varies according to the length of the vara. The Costa Rican manzana equals 0.698 896 hectares or about 1.727 acres. Very similar units are used in Guatemala, Honduras, and Nicaragua. The word manzana means an apple. The unit is probably related to manzanar, orchard.
marathon
a traditional unit of distance used in athletics. The length of a marathon is exactly 26 miles 385 yards, or about a millimeter less than 42.195 kilometers. A story is told sometimes that the marathon is the exact distance run by Pheidippides in 490 BC to bring to Athens the news of the Greek victory over the Persians at the Battle of Marathon. Unfortunately, the actual distance from Marathon to Athens is only about 36.75 kilometers. In fact, 26 miles 385 yards is simply the distance run in the marathon event at the 1908 Olympics in London. The marathons at the Olympic Games were of varying lengths until the Paris Olympics of 1924, when the International Olympic Committee fixed the 1908 London distance as official.
marc, marco, or mark
traditional units of weight in various countries of Western Europe. In each country the unit equals 1/2 the unit corresponding to the English pound. Thus the French marc equals 1/2 livre, 8 onces or about 244.75 grams; the Spanish marco equals 1/2 libra or about 230 grams; the German mark equals 1/2 pfund or about 280.5 grams; and the English mark equals 8 ounces or 226.8 grams. The English unit was used almost entirely for measuring precious metals.
mas
symbol for milliarcsecond, a unit of angular measure commonly used in astronomy.
maund
a traditional unit of weight in India and throughout South Asia. The maund varied considerably, but during the period of British rule in India it was standardized at about 82.286 pounds or 37.3242 kilograms. The maund is divided into 40 seers. Since 1980, Pakistan has used a metric version of the maund equal to exactly 40 kilograms (88.185 pounds); this would make the seer equal to the kilogram.
maxwell (Mx)
a CGS unit of magnetic flux, equal to 10-8weber. In a magnetic field of strength one gauss, one maxwell is the total flux across a surface of one square centimeter perpendicular to the field. This unit was formerly called the line [2]. The newer name honors the British physicist James Clerk Maxwell (1831-1879), who presented the unified theory of electromagnetism is 1864.
mc-
alternate symbol for micro- (see below). This prefix is often seen in the symbol mcg for the microgram. Occasionally, mc has been used as a symbol for the micron (see below).
MBF or MBM
traditional symbols for 1000 (not one million) board feet, a unit of volume for timber equal to 250/3 = 83.333 cubic feet or 2.360 cubic meters. "BM" stands for "board measure."
MBtuh
a symbol for 1000 Btu (British thermal units) per hour, a unit traditionally used in the U.S. heating and air conditioning industry to state rates of heating or cooling. One MBtuh equals about 0.293 071 kilowatt.
MCF
a traditional symbol for 1000 (not one million) cubic feet, a unit of volume equal to about 28.317 cubic meters.
Mcfd
a symbol for 1000 cubic feet per day, a unit of water flow used by many U.S. water supply companies and agencies. 1 Mcfd = 19.665 liters (5. 195 U.S. gallons) per minute.
MCU
a symbol for milk clotting 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 gelatin digesting units (GDU); 1 MCU equals approximately 2/3 GDU.
mease
a unit of quantity formerly used by fishermen. The mease equals the number of herring in a basket, roughly 620.
mebi- (Mi-)
a binary prefix meaning 220 = 1 048 576. This prefix, adopted by the International Electrotechnical Commission in 1998, is intended to replace mega- for binary applications in computer science. The prefix is a contraction of "megabinary."
meg
informal contraction of "megabyte," used in computer science.
mega- (M-) [1]
a metric prefix meaning 106, or one million. (The form meg- is used before a vowel, as in megohm for one million ohms.) The prefix has also become common in ordinary language, meaning "very large," as in megabucks or megadose. This agrees with the derivation of the prefix from the Greek word for large, megas. See Prefix
mega- (M-) [2]
in measuring the storage capacity of a computer, the prefix mega- formerly meant 220 = 1 048 576 instead of an even one million. By a 1998 resolution of the International Electrotechnical Commission, the new prefix mebi- (Mi-) replaces mega- for 220.
megabar (Mbar)
a metric unit of pressure. The megabar equals one million bars, 100 gigapascals (GPa) or about 14.503 million pounds per square inch. Such intense pressures are found inside the earth or in various advanced scientific experiments.
megabase (Mb)
a unit of genetic information equal to the information carried by 1 million pairs of the base units in the double-helix of DNA; also used as a unit of relative distance equal to the length of a strand of DNA containing 1 million base pairs. In humans, one megabase corresponds approximately to a gene separation of one centimorgan.
megabecquerel (MBq)
a unit of radioactivity equal to one million atomic disintegrations per second or 27.027 microcuries.
megabyte (MB)
this unit of information is very common in the computer world, but it is poorly defined. Often it means 1 000 000 bytes, but sometimes it means 2 20 = 1 048 576 bytes. As if that weren't confusing enough, the 1.44 megabytes stored on "high density" floppy disks are actually megabytes of 1 024 000 bytes each.
megacycle (Mc)
1 million cycles; a term sometimes used as an informal name for the megahertz.
megahertz (MHz)
a common unit of frequency equal to one million per second. Frequencies of radio waves are commonly stated in megahertz.
megajoule (MJ)
a common metric unit of work or energy. The megajoule equals one million joules, which is approximately 737 562 foot pounds, 947.8170 Btu, 238.846 (kilogram) Calories, or 0.277 778 kilowatt hours.
megalerg
a CGS unit of energy equal to 106ergs or 0.1 joule (0.073 756 foot pound). The "l" was added to "mega-erg" to make the unit pronounceable.
megaline
a metric unit of magnetic flux, equal to one million lines [2] or 0.01 weber.
megaliter (Ml or ML)
a metric unit of volume equal to 1000 cubic meters. Commonly used in reservoir and water system management outside the U.S., the megaliter equals 264 172 U.S. gallons or 0.810 713 acre foot.
megameter (Mm)
a metric unit of distance equal to 1000 kilometers or about 621.371 miles. Although it seems that this would be an appropriate unit for longer distances on the earth, the megameter is seldom used.
meganewton (MN)
a metric unit of force equal to one million newtons. One meganewton equals about 101 972 kilograms of force or 224 809 pounds of force. The main engines of the U.S. space shuttle have a maximum thrust of about 2.28 meganewtons.
megaparsec (Mpc)
the longest distance unit in common use, the megaparsec is used by astronomers studying the most distant quasars and galaxies. One megaparsec equals 2.36 million light years or 30.857 x 1018 kilometers (30.857 zettameters). See parsec.
megapascal (MPa)
a common metric unit of pressure. One megapascal equals 10 bars or approximately 145.038 pounds per square inch (lbf/in2 or psi) or 20 885.5 pounds (10.443 U.S. tons) per square foot.
megapixel
a unit used to describe the size or resolution of an image or of a digital camera. One megapixel is one million pixels (picture elements, or "dots"). Thus a rectangular image 1000 pixels by 1000 pixels is comprised of one megapixel.
megapond (Mp)
a metric unit of force equal to 1000 kilograms of force (kgf). The megapond also equals 9806.65 newtons, or 2204.6226 pounds of force in the traditional English system. Although it is considered obsolete, the megapond is still used sometimes by engineers in Europe, especially in Germany.
megaton (Mton or Mt)
a unit of energy used for measuring the energy of an explosion, especially a nuclear explosion. One megaton is the amount of energy released by the explosion of one million (short) tons of TNT. This is approximately 4.18 x 1015joules (4.18 petajoules) or roughly 4 trillion Btu.
megatonne (Mt)
a metric unit of mass or weight equal to one million metric tons (tonnes) or about 2.2046 billion pounds.
megawatt (MW)
a common metric unit of power. One megawatt is equal to one million watts, about 1341.02 horsepower, or 947.817 Btu per second.
megawatt hour (MW·h)
a metric unit of energy, especially electrical energy. One megawatt hour equals exactly 3.6 gigajoules (GJ), about 3.412 million Btu, or about 2.655 billion foot pounds.
megohm
a common unit of electric resistance equal to one million ohms.
megayear
A common unit of time equal to one million years.
meile
a traditional distance unit in German speaking countries, the meile is much longer than the mile units of western Europe. Typically the meile was equal to 4000 klafters (fathoms) or 24 000 fuss (German feet). In Austria this came to 7586 meters (4.714 miles); in northern Germany it was 4.6805 miles or 7532.5 meters. A version of the meile called the geographische meile was defined to equal exactly 4 (Admiralty) nautical miles (24 320 feet, 4.6061 miles, or 7412.7 meters).
Mercalli intensity scale
an empirical scale for rating the effects of an earthquake, as opposed to its strength (see magnitude [2] above). Mercalli estimates are stated as Roman numerals (I-XII) to avoid confusion with magnitude estimates on the Richter scale. The scale is named for the Italian geologist Giuseppe Mercalli, who devised the first version in 1902; the modified version used in the U.S. and Canada was developed by Charles F. Richter in 1956.
-merous
an ending meaning "-parted," added to a number to create an adjective. Thus "8-merous" means "having 8 parts." The suffix, frequently used by botanists, is derived from the Greek meros, "part."
mesh
a traditional unit used to measure the fineness of woven products such as fishing nets, fencing fabric, window screening, etc., equal to the number of strands per inch.
met
a unit of metabolism. Metabolism, the sum of all the processes going on in the body to sustain life, is measured in units of power expended per unit of body surface area. One met is the metabolism of a seated, resting person, equal to about 58.15 watts per square meter (W/m2) or 13.89 calories per second per square meter (cal/m2s) regardless of the person's size. Measurements of human metabolism generally fall in the range 0.8-3.0 met, although athletes can achieve 10 met or more.
meter or metre (m)
the fundamental metric and SI unit of distance. Originally, the meter was defined to be one ten-millionth of the distance between the Equator and the North Pole. Because the Earth is difficult to measure (not to mention the many irregularities in its shape!), this is not a practical definition. For a long time, the meter was the length of an actual object, a bar kept at the International Bureau of Weights and Measures in Paris. In recent years, however, the SI fundamental units (with one exception) have been redefined in abstract terms which can (in principle, at least) be reproduced to any desired level of accuracy in a well-equipped laboratory. For the meter, the 17th General Conference on Weights and Measures in 1983 adopted this "simple" definition: the meter is defined as that distance which makes the speed of light in a vacuum equal to exactly 299,792,458 meters per second. The speed of light in a vacuum is one of the fundamental constants of nature. Experiments previously made to measure the speed of light are now reinterpreted as measurements of the meter instead. The meter is equal to approximately 1.0936133 yards, 3.280840 feet, or 39.370079 inches. Its name comes from the Latin metrum and the Greek metron, both meaning "measure." The unit is spelled meter in the U.S. and metre in Britain; there are many other spellings in various languages (see Spelling of Metric Units).
meter-atmosphere
another name for the atmo-meter.
methuselah
a large wine bottle holding about 6 liters, 8 times the volume of a regular bottle.
Metonic cycle
a unit of time equal to 19 years, used in astronomy in predicting the phases of the Moon. By coincidence, 19 years is equal to 6939.602 days and 235 lunar months is equal to 6939.689 days, just 125 minutes longer. As a result, the phases of the Moon repeat almost exactly after 19 years. (Since 19 years can contain either 3 or 4 leap days, the recurrence isn't always exact as to the day of the month.) Lunar calendars, such as the Chinese and Jewish calendars, all share this 19-year cycle. The cycle is named for the ancient Greek astronomer Meton, who used it for predictions in the 5th century BC.
metric carat
the current internationally-recognized carat, equal to exactly 200 milligrams.
metric grain
a unit of mass sometimes used by jewelers, equal to 50 milligrams or 1/4 carat.
metric horsepower
a unit of power, defined to be the power required to raise a mass of 75 kilograms at a velocity of 1 meter per second. This is approximately 735.499 watts or 0.986 32 horsepower. The unit is also known by its French name cheval vapeur or its German name pferdestärke.
metric hundredweight
an informal unit of mass equal to 50 kilograms or approximately 110.231 pounds. This is quite close to the traditional British hundredweight of 112 pounds. This unit is also known by its German name, the zentner, or (in English) the centner.
metric mile
an informal unit of distance used mostly in athletics. The metric mile is equal to 1500 meters or 1.5 kilometers (approximately 0.932 057 statute mile or 4921.26 feet). In U.S. high school competition the term is sometimes used for a race of 1600 meters (0.994 194 miles or 5249.34 feet).
metric quintal
a unit of mass equal to 1 decitonne, 100 kilograms or about 220.462 pounds. See quintal for a more complete description.
metric slug
see TME.
metric ton (t or MT)
an alternate name for the tonne. In the United States, the Department of Commerce strongly recommends that the tonne be called the metric ton to distinguish it clearly from the traditional American ton. The proper symbol for the unit is simply t.
Mgd
an abbreviation for millions of gallons per day (Mgal/d), a unit used in reservoir management to express the rate at which water is withdrawn, or could be withdrawn, for drinking or for some other purpose. 1 Mgd equals approximately 3.785 43 megaliters per day, or 3785.43 cubic meters per day, or 133 681 cubic feet per day.
_mg/dl
symbol for milligram per deciliter, the unit used in medicine to measure the concentration of cholesterol and other substances in the blood. 1 mg/dl equals 0.01 grams per liter (g/L).
_mg/kg
symbol for milligram per kilogram, a unit used in medicine to measure dosage rates. 1 mg/kg is equivalent to 10-6 g/g or 1 part per million based on the patient's body weight.
mho
a unit of electric conductance, equal to 1 divided by the impedance of a circuit as measured in ohms. This makes the mho the same as the siemens, except that the mho is used in circuits in which current is retarded by self-inductance as well as by resistance.
mic
an informal name for the microgram, pronounced "mike."
micro- (µ- or mc-)
a metric prefix meaning 10-6 (one millionth). The prefix comes from the Greek prefix mikro-, meaning small. The prefix is abbreviated mc- when the Greek letter mu (µ) is not available. See Prefix
microampere (µA)
a unit of electric current equal to 10-6 ampere.
microbar (µbar)
a CGS unit of pressure equal to 0.001 millibar, 0.1 pascal, 1 dyne per square centimeter (1 barye), or about 0.002 089 pounds per square foot. The microbar is used commonly in acoustics and sound engineering. See Converter.
microcurie (µCi)
a common unit of radioactivity. The microcurie equals 10-6curie or 37 kilobecquerels; this corresponds to a radioactivity of 37 000 atomic disintegrations per second.
microdegree (µ deg) [1]
a unit of angle measure equal to a millionth of a degree or exactly 36 milliarcseconds.
microequivalent (µEq orµeq)
a unit of relative amount of substance equal to 10-6equivalent weight. This unit is used, for example, in stating the concentrations of ions in drinking water.
microfarad (µF)
a common unit of electric capacitance equal to 10-6farad.
microgram (µg or mcg)
a metric unit of mass equal to 0.001 milligram (mg). Ingredients of drugs and vitamins are often stated in micrograms.
microinch (µin)
a traditional unit of distance equal to 10-6 inch, 0.001 mil, or 25.4 nanometers (nm). The microinch is used rather widely to state the roughness of optical surfaces, precise tolerances in machining, and for other industrial purposes.
microliter (µl, µL, mcl, or mcL)
a metric unit of volume equal to 0.001 milliliter or 1 cubic millimeter (mm3). Microliters are used in chemistry and medicine to measure very small quantities of liquid. This unit has also been called the lambda.
micrometer (µm)
a common metric unit of distance equal to 0.001 millimeter or about 0.039 370 mil.
micromicro- (µµ-)
an obsolete metric prefix denoting 10-12. The prefix has been replaced by pico- (p-).
micromicron (µµ)
a former name for a millionth of a micron, that is, 10-12 meter. The name bicron was also used for this unit, which is now called the picometer (pm).
micromole (µmol)
a unit of amount of substance equal to a millionth of a mole (see below). This unit is used very commonly in biochemistry, since a mole of a large organic molecule can be quite a large amount.
micron (µ)
a metric unit of distance equal to one millionth of a meter. "Micron" is simply a shorter name for the micrometer. In 1968 the CGPM decided to drop the micron as an approved unit and recommend that micrometers be used instead. Microns, however, are still in common use.
microrad (µrad)
a unit of radiation dose equal to a millionth of a rad or 10 nanograys.
microradian ( µrad)
a unit of angle measure equal to 10-6 radian. The microradian equals about 0.208 533 milliarcseconds (mas).
microrem (µrem)
a unit of effective radiation dose equal to a millionth of a rem or 10 nanosieverts. Doses in this range are much smaller than those provided by natural sources of radioactivity in the environment.
microsecond (µs orµsec)
a unit of time equal to a millionth of a second.
microsievert (µSv)
a unit of radiation dose equal to 10-6sievert or 0.1 millirem. The radiation doses resulting from exposure to natural sources such as radon gas in the atmosphere are often measured in this unit.
microstrain
a common engineering unit measuring strain. An object under strain is typically deformed (extended or compressed), and the strain is measured by the amount of this deformation relative to the same object in an undeformed state. One microstrain is the strain producing a deformation of one part per million (10-6).
microtesla ( µT or mcT)
a common unit of magnetic field intensity equal to 10-6tesla. The unit is widely used to measure the strength of electromagnetic fields generated by powerlines or electronic equipment. By comparison, the strength of the Earth's own magnetic field at the surface is about 50 microteslas. One microtesla equals 0.01 gauss.
miglio
the traditional Italian mile. The miglio equals 1628 yards, which is 0.925 English mile or about 1488.6 meters. This is 32 yards (29.3 meters) shorter than the classical Roman mile.
miil or mijl
alternate spellings for the Scandinavian mil [4] (see below).
mil [1]
a unit of distance equal to 0.001 inch: a "milli-inch," in other words. Mils are used, primarily in the U.S., to express small distances and tolerances in engineering work. One mil is exactly 25.4 microns, just as one inch is exactly 25.4 millimeters. In Britain this unit is usually called the thou.
mil [2]
a unit of angle measure, used in the military for artillery settings. During World War II the U. S. Army often used a mil equal to 1/1000 of a right angle, 0.1 grad, or 0.09°. More recently, various NATO armies have used a mil equal to 1/1600 right angle, or 0.05625°. In Britain, an angular mil is often understood to mean 0.001 radian, which is about 0.0573°. (With this last definition, an object one meter wide subtends an angle of one mil at a distance of one kilometer.)
mil [3]
an informal British unit of volume equal to the milliliter (mL).
mil [4]
in Scandinavia, the mil, pronounced like "meal" in English, is a traditional distance unit considerably longer than Roman or English miles. In Denmark, the traditional mil was 24 000 Danish feet, which is 4.6805 miles or 7.5325 kilometers (this is the same as the north German meile; see above). The Danish mil has sometimes been interpreted as exactly 7.5 kilometers (4.6603 miles). In Sweden, the traditional mil was 36 000 Swedish feet, which is 6.641 miles or 10.687 kilometers. In Sweden, Norway, and Iceland, the mil (mila in Iceland) is now interpreted as a metric unit equal to exactly 10 kilometers (6.2137 miles).
mil [5]
an alternate spelling of the mill (see below).
mile (mi)
a traditional unit of distance. The word comes from the Latin phrase milia passuum, "thousands of paces," because originally a mile was the distance a Roman legion could march in 1000 paces (right foot, left foot -- that's one pace). The Roman mile appears to have been equal to about 4980 feet or 1517 meters. Miles of approximately this length were used throughout Western Europe. In 1592 the British Parliament defined the statute mile to be 320 rods, 1760 yards or 5280 feet. This unit replaced various miles previously used in Britain, including the traditional Scottish mile of 1980 yards and the Irish mile of 2240 yards. In metric terms, 1 statute mile is exactly 1609.344 meters. In athletics, 1500 meters is sometimes called a metric mile. See also nautical mile. See Length
miles per gallon (mi/gal or mpg)
the unit customarily used in the United States to measure the fuel consumption rate of motor vehicles. 1 mile per gallon equals about 0.4252 kilometers per liter. In most other countries, however, the usual measure of fuel consumption is liters per 100 kilometers;m miles per U.S. gallon is equal to 235.215/m liters per 100 kilometers. Similarly, p miles per British imperial gallon is equal to 282.481/p liters per 100 kilometers. See Fuel Consumption
miles per hour (mi/hr or mph)
a traditional unit of velocity. One mile per hour equals exactly 22/15 feet per second, approximately 1.609 kilometers per hour, or about 0.447 meters per second. See Velocity
mil-foot
a mil-foot is a section of wire one foot long and one mil in diameter; this would be a unit of volume equal to about 0.0377 cubic inches or 0.6178 cubic centimeters. However, the unit is used primarily in statements of resistivity in ohms per mil-foot or of density in pounds per mil-foot. The unit is also called the circular mil-foot.
milha
the traditional Portuguese mile, one of the "longest miles" of all at 2282.75 yards (1.297 statute miles or 2087.3 meters).
military pace
another name for a step. In the U.S. Army, the military pace is defined to be exactly 30 inches (76.2 centimeters) for ordinary "quick time" marching and 36 inches (91.44 centimeters) for double time marching. The same definitions are generally used by marching bands.
mill
a unit of quantity or of proportion, equal to 0.001. When Congress established the U. S. monetary system in 1791, it provided for 10 mills to the cent and 100 cents to the dollar; thus the mill was an amount of money equal to $0.001. Although the mill is obsolete now as a monetary unit, it has come to represent a one thousandth part as a proportion. Many towns in the United States set their property tax rates in mills, for example.
milla
the traditional Spanish mile, equal to 5000 pies (Spanish feet) or 8 estadios. This is about 1392 meters, 4567 feet, or 0.865 statute mile.
mille [1]
the traditional French mile, equal to 1000 toises. This is equal to about 6394.4 feet, 1.211 statute mile, or 1949 meters.
mille [2]
the Latin word for 1000, sometimes used in English in very learned or literary contexts.
millenary
a unit of quantity equal to 1000.
millennium
a traditional unit of time equal to 1000 years.
milli- (m-)
a metric prefix meaning 0.001 (one thousandth). The prefix was coined from the Latin number mille, one thousand. See Prefix
milliampere (mA)
a common unit of electric current equal to 0.001 ampere.
milliarcsecond (mas)
a unit of angular measure commonly used in astronomy. One milliarcsecond is equal to 16.6667 microdegrees.
milliard [1]
a unit of quantity equal to 109, which is what Americans call a billion. See Using Numbers and Units for more on the "billion" controversy.
milliard [2]
a unit of volume used by engineers to describe a large quantity of water. One milliard equals one cubic kilometer, which is 1 billion (109) cubic meters or about 810 767 acre feet.
millibar (mb)
a common metric unit of atmospheric pressure, equal to 0.001 bar, to 100 pascals, to 1000 dynes/cm2, to about 0.0295 inches (0.7502 millimeters) of mercury, or to about 0.014504 lb/in2. A millibar is the same thing as a hectopascal (hPa), and some weather agencies have replaced the millibar with the hectopascal in an effort to conform with the SI. However, many meteorologists resist this change and continue to use millibars. In fact, the most appropriate SI unit for atmospheric pressure is probably the kilopascal (10 millibars). See Converter.
millicurie (mCi)
a common unit of radioactivity. One millicurie represents radioactivity at the rate of 37 million atomic disintegrations per second, that is, 37 megabequerels.
millidegree (mdeg) [1]
a unit of angle measure equal to 0.001° or exactly 36 arcseconds.
millidegree (mdeg) [2]
a unit of temperature equal to 0.001°, usually meaning 0.001 °C.
milliequivalent (mEq or meq)
a unit of relative amount of substance commonly used in chemistry. One mEq equals 0.001 equivalent weight.
millier
a former name for the tonne or metric ton. This name, obsolete now, was used in Britain to avoid confusion with the British long ton.
millifarad (mF)
a common unit of electric capacitance equal to 0.001 farad.
milligal (mGal or mgal)
a unit of acceleration used in geology to measure subtle changes in gravitational acceleration. One milligal equals 10 micrometers per second per second, or 10-5 meters per second per second. The unit should really be called the milli galileo.
milligauss (mG)
a unit of magnetic flux density equal to 0.001 gauss or 0.1 micro tesla.
milligram (mg)
a common metric unit of mass. One milligram equals approximately 0.015432 grain or 35.274 x 10-6 ounce.
millihenry (mH)
a common metric unit of electric inductance equal to 0.001 henry.
millihg
an informal name (pronounced "millig") for the millimeter of mercury (see below).
millijoule (mJ)
a metric unit of work or energy equal to 0.001 joule or 104ergs.
milli-k
a unit used in nuclear engineering to describe the "reactivity" of a nuclear reactor. One milli-k is a reactivity of 0.001 or 0.1%; the origin of the name is that k is a common symbol for reactivity. This unit was introduced in the Canadian nuclear power industry. For a discussion of reactivity, see inhour.
millilambert (mLb)
a common metric unit of illumination equal to 10 lux.
milliliter (ml or mL)
a common metric unit of volume. One milliliter equals one cubic centimeter or approximately 0.061 023 7 cubic inch or 16.231 U.S. minims (see below).
millimass unit (mu or mmu)
a unit of mass equal to 0.001 atomic mass unit, used in physics and chemistry. This unit is also called the millidalton. The millimass unit is an SI unit, but its proper SI symbol is mu, not the older symbol mmu.
millimeter (mm)
a common metric unit of distance. One millimeter equals about 0.039 370 inch or 39.370 mils. See Converter.
millimeter of mercury (mm Hg)
a unit of pressure equal to the pressure exerted at the Earth's surface by a column of mercury 1 millimeter high. When a traditional mercury barometer is used, the pressure is read directly as the height of the mercury column in millimeters. One millimeter of pressure is equivalent to approximately 0.03937 in Hg, 0.01933 lb/in2, 1.333 millibars, or 133.3 pascals. In medicine, blood pressure is traditionally recorded in mm Hg. (In engineering, the millimeter of mercury is often replaced by the torr, the two units being equal to within 1 part per million.) In the symbol mm Hg, Hg is the chemical symbol for mercury. See Converter.
millimeter of water (mmH2O, mm WC, mm CE, mm WS)
a unit of pressure equal to the pressure exerted at the Earth's surface by a column of water 1 millimeter high. This is a small pressure, about 9.8067 pascals, 0.098 067 millibars, 0.03937 inch of water, or 0.204 pounds per square foot. The French symbol is mm CE (colonne d'eau), and the German symbol is mm WS (Wassersäule).
millimicro- (mµ-)
an obsolete metric prefix denoting 10-9 or one billionth. This prefix has been replaced by nano- (n-).
millimicron (mµ)
a former metric unit of distance equal to 0.001 micron or 10-9 meter. The millimicron has been replaced by its equivalent, the nanometer (nm).
millimole (mmol)
a very common unit of amount of substance equal to 0.001 mole (see below).
milline
a traditional unit of advertising. One milline equals the width of a line of "agate" type (5.5 points, or about 2 mm) times the width of a column times one million copies of the publication.
millinewton (mN)
a metric unit of force equal to 0.001 newton, 100 dynes, or about 0.101 972 gram of force (gf).
millinile
a unit used in British nuclear engineering to describe the "reactivity" of a nuclear reactor. One millinile is a reactivity of 10-5. For a discussion of reactivity, see inhour.
millioersted (mOe)
a name sometimes used for the milligauss as a unit of magnetic flux density.
milliosmole (mOsm)
a unit of osmotic pressure, equal to 0.001 osmole, commonly used in biology and medicine.
millipascal second (mPa·s)
an SI unit of dynamic viscosity equal to the centipoise (cP). This unit is gradually replacing the centipoise in many contexts.
millipoise (mP, mPs, or mPo)
a metric unit of dynamic viscosity equal to 0.001 poise or 0.1 millipascal second (mPa·s).
millirad (mrad)
a unit of radiation dose equal to 0.001 rad or 10 micro grays.
milliradian (mrad)
a unit of angle measure equal to 0.001 radian. The milliradian equals about 0.057 296°, 3.437 75 arcminutes, or 3" 26.265'.
millirem (mrem)
a common unit of radiation dose equal to 10 microsieverts (µSv). A millirem is roughly the radiation dose you would receive from wearing a luminous dial watch for a year.
millisecond (ms or msec)
a common unit of time equal to 0.001 second.
millisievert (mSv)
a unit commonly used to measure radiation dose. One millisievert equals 0.001 sievert or 0.1 rem.
millivolt (mV)
a common unit of electric potential equal to 0.001 volt.
milliwatt (mW)
a common unit of power equal to 0.001 watt.
mina
a historic unit of weight, originating in Babylonia and used throughout the eastern Mediterranean. The mina is roughly comparable to the pound, but over the centuries it varied quite a bit. In Babylonian times it was a large unit, roughly 2 pounds, almost as much as a kilogram. The Hebrew mina, frequently mentioned in the Bible, is estimated at 499 grams (1.10 pounds). The Greek mina was equal to 431 grams (0.95 pound). In Biblical times the mina was equal to 60 shekels, and there were 50 minas in a talent.
miner's inch
a traditional unit of water flow in the western United States. The unit originally represented streamflow through an opening one inch (25.4 mm) square at a specified distance below the surface of the water; this distance varied from 4 to 6 inches. In Idaho, Kansas, Nebraska, New Mexico, North and South Dakota, Utah, and Washington the miner's inch is legally defined to equal 9 gallons per minute or 1.2 cubic feet per minute (about 34.07 liters per minute). In Arizona, California, Montana, Nevada, and Oregon the definition is 1.5 cubic feet per minute (42.48 L/min). In Colorado, the legal equivalent is 1.5625 cubic feet per minute (44.25 L/min). See also water inch.
minim (m or min) [1]
a traditional unit of volume used for very small quantities of liquids. In pharmacy, the term drop traditionally meant the same thing as 1 minim. The minim is defined to be 1/60 fluid dram or 1/480 fluid ounce. The U. S. minim is equal to about 0.003 760 cubic inch or 0.061610 milliliter, while the British minim is equal to about 0.003612 cubic inch or 0.059194 milliliter. As you might guess, the word comes from the Latin minimus, small.
minim [2]
a unit of relative time in music equal to 1/2 whole note (a half note) or 1/4 breve. See Musical Scales
minute (min or ') [1]
a unit of time equal to 60 seconds or to 1/60 hour. The word comes from the Latin word minutus, meaning "small." The SI specifies min as the symbol for the time unit and ' as the symbol for the arcminute (see below).
minute (' or m) [2]
a unit of angular measure equal to 60 arcseconds or to 1/60 degree. This unit is often called the arcminute to distinguish it from the minute of time. There are 21 600 arcminutes in a circle. The SI defines min as the symbol for the time unit (see above) and recommends ' as the symbol for the arcminute. The international standard ISO 31 recommends that angles be stated in degrees and decimal fractions of the degree, without use of arcminutes and arcseconds.
minute (' or m) [3]
a unit of longitude used in astronomy. Astronomers measure longitude (or "right ascension", as they call it) in time units by dividing the equator into 24 hours instead of 360 degrees. This makes 1 minute of longitude or right ascension equal to 15 arcminutes.
minyan
a traditional Hebrew unit of quantity equal to 10, the number of males aged 13 or over required for a Jewish worship service.
mips
a unit of computing power equal to one million instructions per second. An "instruction" is a single program command to the computer's central processor. In a particular computer, there is a definite relationship between the rate at which instructions are processed, in mips, and the "clock speed" of the processor, measured in megahertz (MHz). However, this relationship varies considerably between computers, so it is usually not meaningful to compare the mips rates of dissimilar machines.
MIU
symbol for one million international units. Dosages of certain drugs, such as various forms of interferon, are commonly stated in this unit.
mkono
a traditional unit of distance in East Africa, standardized under British rule as 1/2 yard (18 inches, or 45.72 centimeters).
MM
an abbreviation for one million, seen in a few traditional units such as those listed below. The abbreviation is meant to indicate one thousand thousand, M being the Roman numeral 1000. However, MM actually means 2000, not one million, in Roman numeration.
m/m
an abbreviation for "by mass," used in chemistry and pharmacology to describe the concentration of a substance in a mixture or solution. 2% m/m means that the mass of the substance is 2% of the total mass of the solution or mixture.
MM Btu
traditional symbol for one million Btu (about 1.055 057 gigajoules (GJ)), a unit used widely in the energy industry. This unit is also called the dekatherm.
MMCF
a symbol for one million cubic feet (28 316.85 m3, or 28.316 85 megaliters).
MM scfd
symbol for one million standard cubic feet per day, the customary unit for measuring the production and flow of natural gas. "Standard" means that the measurement is adjusted to standard temperature (60 °F or 15.6 °C) and pressure (1 atmosphere).
Mohs hardness scale
a 1-10 scale for estimating the hardness of a mineral, introduced by the German geologist Friedrich Mohs in 1812. To apply the scale, one attempts to scratch the mineral with standard minerals assigned hardness numbers as follows:
  1. diamond 10
  2. corundrum 9
  3. topaz 8
  4. quartz 7
  5. orthoclase 6
  6. apatite 5
  7. flourite 4
  8. calcite 3
  9. gypsum 2
  10. talc 1

If, for example, the mineral is scratched by quartz but not by orthoclase, then its hardness is between 6 and 7.

molal (m), molar (M)
these notations, traditionally used by chemists to describe the concentration of chemical solutions, often appear to be units of measurement. It's easy to get them confused. The term "molal" describes the concentration of a solution in moles per kilogram of solvent (mol/kg), while "molar" describes a concentration in moles per liter (mol/L). A solution described as 1.0 µM has a concentration of 1.0 µmol/L. These units are not approved by the General Conference on Weights and Measures. Their use is declining, but still substantial.
molar volume
a unit used by chemists and physicists to measure the volumes of gases. The behavior of gases under ordinary conditions (not at very high pressures or very low temperatures) is governed by the Ideal Gas Law. This law says that the volume V of a gas is related to its temperature T and pressure P by the formula PV = nRT, where n is the number of moles of gas present and the gas constant R equals 8.314 joules per mole per kelvin. The molar volume is the volume one mole of gas occupies at standard temperature (273.16 kelvins, or 0 °C) and standard pressure (1 atmosphere, or 101.325 kilopascals). The molar volume is equal to 22.414 liters or 0.7915 cubic foot. (Occasionally the term "molar volume" is used for the volume occupied by a mole of a substance which is not a gas; in such cases the molar volume will be different for each substance.)
mole (mol)
the SI fundamental unit of the amount of a substance (as distinct from its mass or weight). Moles measure the actual number of atoms or molecules in an object. An alternate name is gram molecular weight, because one mole of a chemical compound is the same number of grams as the molecular weight of a molecule of that compound measured in atomic mass units. The official definition, adopted as part of the SI system in 1971, is that one mole of a substance contains just as many elementary entities (atoms, molecules, ions, or other kinds of particles) as there are atoms in 12 grams of carbon-12 (carbon-12 is the most common atomic form of carbon, consisting of atoms having 6 protons and 6 neutrons). The actual number of "elementary entities" in a mole is called Avogadro's number after the Italian chemist and physicist Amedeo Avogadro (1776-1856). Careful measurement determines Avogadro's number to be approximately 6.02214199 x 1023 entities per mole. In the American system of naming big numbers, that's 602 sextillion 214 quintillion 199 quadrillion, give or take about 50 quadrillion.
momme
a traditional Japanese weight unit corresponding to the Chinese mace (see above). Jewelers continue to use the momme to measure the weight of cultured pearls; for this purpose it equals exactly 3.75 grams or 0.132 ounce.
month (mo or mon) [1]
a unit of time marked by the revolution of the Moon around the Earth. In many traditional societies the appearance of the first tiny crescent moon after the New Moon signaled the start of the month. This start of the month, based on the first appearance of the Moon, is still proclaimed from mosques in Islamic countries. Thus the lunar month is defined as the average interval between two successive moments of New Moon. Astronomers call this period the synodic month. Its length is 29.530 59 days. See Saros, Lunar Month.
month (mo or mon) [2]
a civil unit of time equal to approximately 1/12 year, but varying from 28 to 31 days. The Sun and the Moon are our traditional time keepers, but they are badly out of step with each other. A solar year equals approximately 12.368 lunar months. The large fraction in this number makes it difficult to design a calendar with a whole number of months in each year. There are at least three solutions to the problem:

[i] Use leap months. In the traditional Chinese and Jewish calendars most years have 12 months, but some have a 13th month. In these luni-solar calendars the length of the year varies from 354 to 384 days.

[ii] Define 12 synodic lunar months to be a year and don't worry about the length of the year. This is the solution of the Islamic calendar. Since the Islamic year has only 354 or 355 days, its length does not match the cycle of the seasons.

[iii] Observe the solar year and let the months be 12 arbitrary periods; don't worry about the Moon. This is the solution adopted by the Romans, who established the civil calendar we use today. In this calendar, all months have 30 or 31 days except the second month, February. February has 28 days in ordinary years and 29 in leap years. See also year [2].

morgan (M)
a unit of genetic separation used in genetics and biotechnology. If two locations on a chromosome have a 100% probability of being separated during recombination in a single generation, then the distance between those locations is one morgan. In practice, measurements are made in centimorgans. The unit honors the American geneticist Thomas Hunt Morgan (1866-1945), who received the Nobel Prize for Medicine in 1933 for his pioneering work in studying the genetics of the fruit fly Drosophila.
morgen
a traditional unit of land area in Northern Europe. "Morgen" means "morning," and most likely the unit arose as the area a yoke of oxen could plow in one morning. The Dutch morgen, also used in Dutch colonies including old New York, equals about 2.10 acres or 0.850 hectare. In South Africa, this unit was defined to equal 10 246 square yards, which is 2.1169 acres or 0.8567 hectare. In Scandinavia and northern Germany, the morgen is a smaller unit equal to about 0.63 acre or 0.25 hectare.
mu or mou
a traditional unit of land area in China. The traditional mu is about 675 square meters or 800 square yards. However, the colonial customs authorities used a larger mu equal to 8273.75 square feet, 919.3 square yards, or 768.65 square meters. In modern China, the mu is often reckoned to be exactly 1/15 hectare, which is 666.667 square meters or 797.327 square yards. See Area
mud
a traditional Dutch unit of volume for grains and other dry commodities. Originally varying from market to market, the unit was declared equal to the hectoliter (about 3.5315 cubic feet or 2.838 U.S. bushels) when the metric system was introduced in the Netherlands. With this definition it is still in use.
mug
an informal contraction of "metric slug". See TME.
mutchkin
a traditional Scottish unit of liquid volume. The mutchkin is about 15 British fluid ounces, which is about 426 milliliters or almost exactly 0.9 U.S. pint.
mwe
abbreviation for meter of water equivalent, a unit used in nuclear physics to describe the shielding around a reactor, accelerator, or detector. 1 mwe of any material (such as rock, gravel, etc.) is a thickness of that material providing shielding equivalent to one meter of water.
MWe, MWt
symbols used in the electric power industry to describe the size of generating plants. MWe is the symbol for the actual output of a generating station in megawatts of electricity; MWt is used for the heat energy, or thermal output, required to operate the generators. Thermal output is typically about three times the electric output.
myria- (my-)
a metric prefix meaning 10,000. This prefix was part of the original metric system of 1795 and was used throughout the nineteenth century, but it is now considered obsolete. The ancient Greek word myrios means countless, without number. This was modified by later Greeks to form a word myrioi meaning ten thousand. The word myriad, generally used today to mean an indefinitely large number, originally meant the number 10,000.
myriagram (myg)
a metric unit of mass equal to 10,000 grams or 10 kilograms (about 22.046 pounds). Although it is considered obsolete now, the myriagram was a useful unit comparable to the English quarter or Spanish arroba.