D

D-
an incorrect symbol for the metric prefix deka- or deca-, seen in combinations such as DL (dekaliter) or dtH (dekatherm). The correct prefix is da-.
dag
symbol for the dekagram (10 grams; see entry below), a common unit of mass in metric countries.
daily value (DV)
a unit of nutrition used in the United States. The federal government establishes recommended daily amounts of various nutrients, both "good" ones like vitamins and "bad" ones like fat and sodium. These so-called daily values are based on a hypothetical person, male or female, who requires a diet of 2000 Calories per day. The results are approximate at best, since nutritional needs vary with age, sex, and other factors. Food packages generally carry nutritional lables specifying the amount of each nutrient contained in a standard serving, expressed as a percentage of the daily value (%DV).
dal or daL
symbol for the dekaliter (10 liters; see entry below), a common metric unit of volume.
dalton (Da or D)
an alternate name for the unified atomic mass unit (u or amu). The dalton is often used in microbiology and biochemistry to state the masses of large organic molecules; these measurements are typically in kilodaltons (kDa). It seems necessary to have such a unit, since "kilo-amu" would be such a clumsy name. The SI accepts the dalton as as alternate name for the unified atomic mass unit and specifies Da as its proper symbol. The unit honors the English chemist John Dalton (1766-1844), who proposed the atomic theory of matter in 1803.
dan
a traditional Chinese weight unit, previously spelled tan in most English works. During the European colonial era the unit was equal to 100 catties or 133.333 pounds. In modern China the dan is equal to 100 jin, which is exactly 50 kilograms (110.231 pounds). The dan is the Chinese equivalent of the European quintal or hundredweight.
daN
symbol for the decanewton or dekanewton (see below under dekanewton), a common metric unit of force.
daraf
a unit of electrical elastance, which measures the ability of an electric potential to charge a capacitor. The daraf is equal to one volt of potential per coulomb of charge (V/C). The name of the unit is "farad" spelled backwards, because the elastance, in darafs, is 1 divided by the capacitance in farads. This unit is not recognized as part of the SI.
darcy
a CGS unit of permeability. Permeability is the extent to which a solid allows the flow of a fluid. This flow depends on the properties of the solid and also on the dynamic viscosity of the fluid and the difference in pressure driving the flow. One darcy is the permeability of a solid through which one cubic centimeter of fluid, having a viscosity of one centipoise, will flow in one second through a section one centimeter thick and one square centimeter in cross section, if the pressure difference between the two sides of the solid is one atmosphere. It turns out that permeability has the same units as area; since there is no SI unit of permeability, square meters are used. One darcy is equal to about 0.98692 x 10-12 square meter. The unit is named for a French scientist, H. Darcy (1803-1858), who did pioneering work in the study of permeability.
dash (ds) [1]
an informal unit of volume used in food recipes. American cookbook editors usually figure 2 dashes to the pinch and 8 pinches to the teaspoon. This makes the dash equal to roughly 0.01 fluid ounce or 0.3 milliliter.
dash (ds) [2]
an informal unit of volume used in bartending. The bartender's dash (or splash) is usually taken to be 1/32 fluid ounce or about 0.92 milliliter.
date
dates of the year are described in many ways in different languages. To avoid confusion in international communications, the International Organization for Standardization (ISO) established (in 1986) International Standard 8601 for representation of dates and times. The ISO 8601 format for dates is YYYY-MM-dd, where YYYY is the year number, MM is the month (01 through 12) and dd the day number. Hyphens, not slashes, must be used as the separators in the representation. Thus 2001-04-05 is the representation for the date called April 5, 2001, in traditional U.S. notation. The international standard covers numeric representations only; it doesn't prohibit traditional statements of the date, using words, in whatever the local style is. Thus "April 5, 2001" remains perfectly acceptable in U.S. usage. However, non-standard numeric representations, such as 04/05/01, should not be used, because these representations are interpreted differently in different countries.
day (d or da)
a traditional unit of time marked by the rotation of the Earth on its axis.
As viewed from outside the Solar System, Earth turns on its axis once in approximately 23 hours 56 minutes 4 seconds, a period astronomers call the sidereal day. The day seems a little longer than that to us, because by the time 23:56:04 has passed, the Earth has moved 1/365 of the way in its orbit around the Sun. This causes the Sun to appear a little farther east than it did at the start of the day. It takes the Earth, on the average, about 3 minutes 56 seconds to turn a bit more and put the Sun back in the same apparent position it occupied at the start of the day.
There's more. The speed of the Earth in its orbit varies, because the Earth moves faster when it is closer to the Sun. This causes the length of an actual day to vary slightly over the course of the year. (Despite what you might think, days are actually longer during the Northern Hemisphere winter and shorter during the Northern Hemisphere summer, because, again despite what you might think, the Earth is actually closer to the Sun during the Northern Hemisphere winter.) Because of these irregularities in the length of actual days, we adjust our clocks to measure the average period of time between two successive noons, that is, between two successive appearances of the Sun on the meridian (the imaginary line joining due north to due south across the sky). Astronomers call this period of time the mean solar day.
By ancient custom we divide the day into 24 hours, or 1440 minutes, or 86 400 seconds. But there is also a small problem with doing this. The second has its own scientific definition based on the frequency of a certain radiation from cesium atoms and having nothing to do with the Earth's rotation. The reason for making this scientific definition of the second is that the length of the mean solar day is not constant from one year to the next. The gravitational attraction of the Moon is very gradually slowing the Earth's rotation, so that each day is a tiny bit longer, about 40 nanoseconds on the average, than the previous day. This lengthening adds one second to the length of the day about every 60 000 years. During the first decades of the 21st century the actual length of the mean solar day will be about 86,400.002 seconds.
For this reason, time as kept by our best clocks runs a bit faster than time as kept by the Earth. To keep clock time and sun time in step (within 0.9 seconds), every so often a leap second must be added at the end of a day. This is done either on June 30 or on December 31. Because the rate of slowing is erratic, the need for these leap seconds cannot be predicted more than a few months in advance.
dB -
a symbol indicating that a measurement is made using a logarithmic scale similar to that of the decibel (see below) in that a difference of 10 dB- corresponds to a factor of 10. In each case, the actual measurement a is compared to a fixed reference level r and the "decibel" value is defined to be 10 log 10(a/r). Many units of this kind have been used and only a few of the more common ones are mentioned in the next entries. In each case the dB symbol is followed by a second symbol identifying the specific measurement. Often the two symbols are not separated (as in "dBA"), but the Audio Engineering Society recommends that a space be used (as in "dB A").
dB A, dB C
units of sound intensity, exactly like the decibel except that before the measurement is made sounds of high and low frequencies, heard poorly or not at all by the human ear, have been filtered out. The letters A and C refer to two filtering methods.
dB c
a unit of signal strength used in electronics, especially in measuring noise levels. The signals are measured relative to the strength of the carrier signal, which is the desired signal. A typical statement might be that a certain noise level is -50 dB c, meaning that the noise is 50 "decibels below carrier" or 10-5 times the carrier signal strength.
dB FS
abbreviation for "decibels full scale," a unit of power as measured by a digital device. A digital measurement has a maximum value M depending on the number of bits used. If the actual power measurement is p, the dB FS value displayed is 20·log 10(p/M) dB FS. Since p cannot exceed M, this reading is always negative.
dB i
a unit measuring the gain of an antenna. The reference level is the strength of the signal that would be transmitted by an isotropic antenna: one radiating equally in all directions. For example, an antenna rated 20 dB i transmits a signal in the desired direction 102 = 100 times stronger than an isotropic antenna.
dB m, dB W
logarithmic units of power used in electronics. These units measure power in decibels above the reference level of 1 milliwatt in the case of dB m and 1 watt in the case of dB W. A power of n watts equals 10 log n dB W; conversely, a power of p dB W equals 10(p/10) watts. The same formulas link dB m to milliwatts. An increase of 10 dB m or 10 dB W represents a 10-fold increase in power. Since 1 watt = 1000 milliwatts, 0 dB W = 30 dB m.
dB rn
a symbol for "decibels above reference noise," a unit measuring noise levels in telecommunications. The usual reference level is -90 dB m, which is equivalent to a power of 1 picowatt (1 pW). For example, 50 dB rn equals -40 dB m.
dB spl
a logarithmic unit of sound intensity as computed from the sound pressure level. The reference level is a pressure of 20 micropascals. If sound waves exert a pressure of P pascals, the sound intensity is 100 + 20·log10(P/2) dB spl.
dB u
a logarithmic unit of power, similar to dB m but computed from voltage measurements. The reference level is 0.775 volts, the voltage which generates a power of 1 milliwatt across a circuit having an impedance of 600 ohms. A voltage of V volts corresponds to a power of 20·log10 (V/0.775) dB u.
dB V
a logarithmic unit of power, similar to dB m but computed from voltage measurements. The reference level is 1 volt. A voltage of V volts corresponds to a power of 20·log10(V) dB V.
deadweight ton (dwt)
a traditional unit of weight or mass used in the shipping industry. The deadweight tonnage of a ship is the difference between its weight when completely empty and its weight when fully loaded. This includes the weight of everything portable carried by the ship: the cargo, fuel, supplies, crew, and passengers. The deadweight ton is traditionally equal to the British ("long") ton of 2240 pounds (1016.047 kilograms). However, more and more often it is being taken to equal the metric ton (exactly 1000 kilograms, or 2204.623 pounds).
debye (D)
a CGS unit of electric dipole moment used in chemistry and physics. A charged molecule can be regarded as a tiny bar with opposite charges at the ends. Such a bar is called a dipole, and its dipole moment is the amount of the charge multiplied by half the length of the bar. Thus the appropriate unit of dipole moment in the SI would be the coulomb meter (C·m). Since this is much too large for molecules, the debye is defined as an electric dipole moment of 10-18 statcoulomb centimeter or 3.33564 x 10-30 coulomb meter. The unit honors the Dutch physicist P.J.W. Debye (1884-1966), who was famous for his research on polar molecules.
deca- (da-)
a metric prefix meaning 10. The prefix is taken from the Latin word for ten, decem. The Greek form deka- is also used, and it has the advantage of avoiding confusion with deci-, one tenth. The International Bureau of Weights and Measures (BIPM) specifies deca- for the prefix and da- for the symbol. In its interpretation of the International System (SI) for U.S. use, the U.S. National Institute of Standards and Technology (NIST) recommends deka- for the spelling of the prefix. The SI allows national variations in the spelling of units and prefixes, but not in symbols. Following NIST recommendations, listings for units beginning deca- will be found below under deka-.
decade [1]
a traditional unit of quantity equal to 10. Also called the decad. In medieval English this unit was anglicized as the dicker (see below).
decade [2]
a traditional unit of time equal to 10 years.
decade [3]
a unit of time equal to 10 days or 1/3 month. The revolutionary governments in France (1790s) and the Soviet Union (1920s) tried to decimalize and secularize the calendar by eliminating the week (with its traditional day of religious observance) and substituting the decade. Both efforts failed. However, the decade survived as a unit of civil time in Russia and as a 10-day unit in some scientific contexts (for example, a 10-day cycle in meteorology is called a decade). Where the decade equals 1/3 month, the three decades of a month are the 1st-10th, 11th-20th, and 21st through the last day of the month.
decade [4]
another name for the bel or the order of magnitude: a logarithmic unit used to compare the sizes of quantities. Two quantities differ by one decade if one is 10 times the other, by two decades if one is 10·10 = 100 times the other, and so on.
decatherm
see dekatherm, below.
decay time [1]
a unit of relative time used in physics. Decay time is similar to half life, but shorter and less familiar. It is the time required for an exponentially decaying process (such as radioactivity) to decrease to 1/e = 36.7879% of its original value. The fraction of activity remaining at time T, if T is measured in decay time units, is simply e-T. The decay time equals 0.693 147 half life.
decay time [2]
a unit of relative time used in various engineering applications. In many cases, it is the time required for a decaying process to decrease to 10% of its original value. However, a variety of definitions are used in different fields.
deci- (d-)
a metric prefix meaning one tenth, or 0.1. The prefix is derived from the Latin word decimus for a tenth. Interestingly the Romans Legions suffered decimation for failure, each ten man was kill. See Prefix
decibar (dbar)
a metric unit of pressure equal to 0.1 bar or 10 kilopascals. One decibar equals 75.006 torr, or 1.450 pounds per square inch (lbf/in 2 or psi). The decibar is often used to measure the pressure of seawater, because an increase of 1 decibar in pressure corresponds closely to an increase of 1 meter in depth.
decibel (dB)
a customary logarithmic unit most commonly used (in various ways) for measuring sound. The human ear is capable of detecting an enormous range of sound intensities. Furthermore, our perception is not linear. Experiment shows that when humans perceive one sound to be twice as loud as another, in fact the louder sound is about ten times as intense as the fainter one. For this reason, sound is measured on logarithmic scales. Informally, if one sound is 1 bel (10 decibels) "louder" than another, this means the louder sound is 10 times louder than the fainter one. A difference of 20 decibels corresponds to an increase of 10 x 10 or 100 times in intensity. The beginning of the scale, 0 decibels, can be set in different ways, depending on exactly which aspect of sound is being measured. For sound intensity (the power of the sound waves per unit of area) 0 decibels is equal 1 pico watt per square meter; this corresponds approximately to the faintest sound which can be detected by a person who has good hearing. A quiet room has a normal sound intensity of around 40 decibels, ten thousand times louder than the faintest perceptible sound, and a thunderclap may have an intensity of 120 decibels, a trillion times louder than the faintest sound. For sound pressure (the pressure exerted by the sound waves) 0 decibels equals 20 micropascals (µPa) RMS, and for sound power 0 decibels sometimes equals 1 picowatt. In all cases, one decibel equals about 0.115 129 neper and d decibels equal d(ln 10)/20 nepers. See also dB- (above).
decigram (dg)
a metric unit of mass equal to 100 milligrams or about 1.5432 grains.
decile
a statistical unit equal to 10 percentiles, or 1/10 of a ranked sample. See percentile. The word is pronounced with a soft "c": des-ile.
deciliter (dl or dL)
a common metric unit of volume equal to 0.1 liter or 100 cubic centimeters. A deciliter contains 6.10237 cubic inches, 3.38140 U.S. fluid ounces, or 3.519 British fluid ounces. The deciliter is similar in size to the gill, an old English unit of volume.
decimeter (dm)
a fairly common metric unit of distance equal to 10 centimeters or 3.9370 inches. The decimeter is very close to the hand, a traditional English unit.
decitex (dtex)
a common metric unit of yarn density equal to 0.1 tex, 0.9 denier (see below), or 0.1 milligram per meter. This unit was previously called the drex.
decitonne (dt or dtn)
a metric unit of mass or weight equal to 100 kilograms (approximately 220.4623 pounds). This unit is becoming common in international trade; it is the same as the Russian centner, the German doppelzentner (see below), and the French metric quintal.
degree (° or deg) [1]
the standard unit of angle measure, equal to 1/360 circle, 60 minutes, 3600 seconds, or about 0.017 453 293 radian.
degree (° or deg) [2]
a unit of distance sometimes used at sea, equal to 60 nautical miles, approximately 69.05 statute miles, or 111.12 kilometers. This distance is the average length of one degree of latitude (that is, the average distance between two lines of latitude 1° degree apart). On the Earth's surface, one degree of latitude actually varies from about 68.7 miles at the Equator to 69.4 miles at the poles, while one degree of longitude is about 69.17 miles at the Equator (shrinking to nothing at the poles!).
degree (° or deg) [3]
any of the various units of temperature (see below). In the U.S., the unmodified unit "degree" generally means the degree Fahrenheit; in other countries, it means the degree Celsius.
degree (° or deg) [4]
a unit measuring the hardness of water. Water is called "hard" if it contains a high concentration of mineral salts, especially calcium carbonate. This concentration can be expressed clearly in parts per million (ppm) or in milligrams per liter (mg/L). Starting in the mid-19th century, several uses of "degree" for water hardness became well established. In the U.S. and Britain, hardness is measured in Clark degrees, named for the scientist who devised the first reliable test for water hardness. The Clark degree is defined as 1 part of calcium carbonate per 70 000 parts of water; this is about 14.3 ppm, 17.1 mg/L, or 1 grain per gallon (gpg), another popular measure of hardness. The French, properly decimal as always, used a degree equal to exactly 10 ppm, while the German degree was equivalent to 17.8 ppm.
degree(° or deg) [5]
various scientific quantities, especially specific gravity and viscosity, have been measured in "degrees" based on the readings of particular instruments. The degree scales of Baumé (see below) and Beck, and many others, have been used in measuring specific gravity, and the scales of Engler and MacMichael are common in the measurement of viscosity. Often these degree scales can't be converted in a simple way to the quantities being measured; one needs the conversion formula or table for the particular instrument. Some of this data can be found in scientific and engineering handbooks, especially older ones.
degree (° or deg) [6]
the percentage of alcohol, by volume, present in a mixture. In winemaking, for example, a 13° wine is 13% alcohol by volume (13% v/v). This unit is also called the degree Gay-Lussac (° GL) after the French chemist Joseph-Louis Gay-Lussac (1778-1850).
degree (deg) [7]
a unit used in mathematics to describe algebraic expressions. The degree of an expression having a single variable is the highest exponent (or power) to which that variable appears. If the expression has more than one variable, then the degree is the highest total exponent of the individual terms, where in each term the exponents of the different variables are added. Thus x2y4z3 is a term of degree 9.
degree (°)[8]
the DIN speed rating for film; see DIN below.
degree (deg) [9]
a unit measuring consanguinity: the extent to which two persons are related to one another by common descent ("by blood," as the usual saying is). Essentially all cultures, societies, and nations have prohibited marriage between closely related individuals, and the determining factor is often the number of degrees of consanguinity between the two persons. If X is an ancestor of Y, the number of degrees of consanguinity between X and Y is the number of descents; thus parent and child are separated by 1 degree, grandparent and grandchild by 2 degrees, and so on. If persons X and Y are not linked by direct descent but do share a common ancestor A, the usual definition is that the number of degrees of consanguinity between X and Y is the total number of descents, counting up from X to A and then back down from A to Y. For example, siblings (brother and sister) are separated by 2 degrees, aunt and nephew by 3 degrees, and first cousins by 4 degrees. This calculation is backed by modern science, because we have learned that persons separated by n degrees of consanguinity share about 1/2n of their genes. See also cousins.
degree API (°API)
a unit invented by the American Petroleum Institute (API) to measure the specific gravity, or one might say the "lightness," of petroleum. Lighter oils are more valuable, so the API scale gives them higher readings than heavier oils. If S is the specific gravity of the petroleum, the API degree rating is equal to (141.5/S) - 131.5 degrees.
degree Balling
see degree Plato, below.
degree Baumé (°B)
a unit of relative density, as read on a type of hydrometer invented by the French chemist Antoine Baumé (1728-1804). Two scales are used, depending on whether the liquid is lighter than water or heavier than water. For lighter liquids, the relative density d in degrees Baumé is related to specific gravity S by the formula d = (144.3 / S) - 144.3; for heavier liquids the formula is d = 144.3 - (144.3 / S).
degree Celsius (°C)
the metric unit of temperature. The Celsius temperature scale is named for the Swedish astronomer and physicist Anders Celsius (1701-1744), who is supposed to be its inventor. It is designed so that the freezing point of water (at one atmosphere of pressure) is 0 °C, while the boiling point is 100 °C. Thus the Celsius degree is defined to be 1/100 of the difference between these two temperatures. (The scale actually used by Celsius was inverted, so that 0 °C was the boiling point of water and 100 °C the freezing point.) In the SI system, the temperature scale has been redefined so that the temperature of the triple point of water (the temperature at which water can exist simultaneously in the gaseous, liquid, and solid states) is exactly 0.01 °C, and the size of the degree is 1/273.16 of the difference between this temperature and absolute zero (the temperature at which all molecular motion ceases). For practical purposes, however, there is no difference between this and the original definition. See also degree Fahrenheit, kelvin. See Converter.
degree centigrade (°C)
an older name for the degree Celsius. The Celsius scale is called "centigrade" because it has 100 (centi-) gradations between the freezing point and boiling point of water. About 1850 scientists began calling the unit "degree Celsius" in order to honor its presumed inventor.
degree day (deg da)
a unit used in meteorology and engineering to measure the demand for heating or cooling. In the United States, it's agreed that 65 °F (18.3 °C) is the critical outside temperature; below this tempeature heating is needed and above it cooling is needed. The number of degree days recorded on a given date is equal to the difference between 65 and the mean temperature for that date (figured as the average of the highest and lowest temperatures). If the mean temperature is above 65, cooling degree days (Cdd) are recorded; if it is below 65, heating degree days (Hdd) are recorded. The total number of heating or cooling degree days recorded in any period of time measures rather accurately the demand during that time for fuel used in heating and cooling buildings.
degree Fahrenheit (°F)
a traditional unit of temperature still used customarily in the United States. The unit was defined by the German physicist Daniel G. Fahrenheit (1686-1736), who also invented the mercury thermometer. Fahrenheit set 0° at the coldest temperature he could conveniently achieve using an ice and salt mixture, and he intended to set 100° at the temperature of the human body. (He was off a little there; normal temperature for humans is between 98 °F and 99 °F.) On this scale, the freezing point of water (at normal sea level atmospheric pressure) turned out to be about 32 °F and the boiling point about 212 °F. Eventually the scale was precisely defined by these two temperatures. 1°F equals 5/9 °C, but in converting between scales we have to be careful to adjust the zero points as well. To convert a temperature in °F to the Celsius scale, we must first subtract 32° and then multiply by 5/9. In the other direction, to convert a temperature in °C to the Fahrenheit scale, we must first multiply by 9/5 and then add 32°. See Converter
degree Gay-Lussac (°GL)
see degree [6] above.
degree Kelvin (°K)
see kelvin. The SI requires that temperatures on the absolute temperature scale be stated in kelvins, not in degrees Kelvin. See Converter.
degree KMW (°KMW)
a unit used in Austria to measure the sugar content of must, the unfermented liquour from which wine is made. One degree KMW is roughly equivalent to 1% sugar by weight or 5° Oe; for the exact conversion see the next entry. KMW is an abbreviation for Klosterneuburger Mostwaage (must scale).
degree Oechsle (°Oe)
a unit used in Germany and Switzerland to measure the sugar content of must, the unfermented liquour from which wine is made. One degree Oechsle (or Öchsle) is roughly equivalent to 0.2% sugar by weight. This unit is related legally to °KMW by the formula:
             °Oe = °KMW * ([.022 * °KMW] + 4.54).
degree Plato (°P)
a unit measuring sugar content, especially of the wort, the unfermented liquour from which beer is made. Named for a German chemist, one degree Plato represents a sugar content equivalent to 1% sucrose by weight. Not all the sugar in a wort is sucrose; the unit standardizes the measurement to the sucrose equivalent. The reading is made with a device called a saccharometer. The degree Balling is a somewhat older unit equivalent (approximately) to the degree Plato.
degree Quevenne (°Q)
a unit measuring the density of milk. 1 °Quevenne represents a difference in specific gravity of 0.001, so, for example, 20 °Q milk has specific density 1.020.
degree Rankine (°R)
a unit of absolute temperature, now replaced by the kelvin. 1 °Rankine represents the same temperature difference as 1 °Fahrenheit, but the zero point of the scale is set at absolute zero. This means the Rankine temperature is 459.67° plus the Fahrenheit temperature. 1 °Rankine is equal to exactly 5/9 kelvin. See Converter. The unit is named for the British physicist and engineer William Rankine (1820-1872).
degree Réaumur (°r)
a unit of temperature formerly used in continental Europe. The Réaumur temperature scale, named for the French scientist René-Antoine Ferchault de Réaumur (1683-1757), is similar to the Celsius scale, but the interval between the freezing and boiling temperatures of water equals 80 °r instead of 100 °C. Therefore, 1 °r equals 1.25 °C or 2.25 °F. See Converter.
deka- (da-)
a metric prefix meaning 10, taken directly from the Greek word for ten, deka. The Latin spelling deca- is also used (see deca-, above). Although the U.S. National Institute for Standards and Technology recommends deka-, deca- is seen frequently in American publications. The spelling deca- is also used in French and in other languages where "k" rarely occurs in the middle of a word. Regardless of spelling, da- is the official symbol. (The symbol dk- for deka- is incorrect, but it is seen fairly often.) See Prefix
dekagram or decagram (dag)
a common metric unit of mass, the dekagram is frequently used in European food recipes. One dekagram is equal to 10 grams, 0.01 kilogram, or 0.35273966 ounce. The symbol dkg sometimes used for this unit is incorrect.
dekaliter or decaliter (daL or dal)
a metric unit of volume equal to 10 liters and comparable to the English peck. The dekaliter is equal to about 2.64172 U.S. liquid gallons, 2.27021 U.S. dry gallons (1.135 10 U.S. pecks), or 2.199 69 British imperial gallon (1.099 85 British pecks). The symbol dkL sometimes used for this unit is incorrect.
dekameter or decameter (dam)
a common metric unit of distance equal to 10 meters (about 32.8084 feet). The symbol dkm sometimes used for this unit is incorrect.
dekanewton or decanewton (daN)
a fairly common metric unit of force equal to 10 newtons. The dekanewton is equal to 1 megadyne, to 1.019 716 kilograms of force (kgf) or kiloponds (kp), to 2.248 09 pounds of force (lbf), and to 72.3301 poundals. In engineering, the dekanewton serves as a convenient replacement for the kilogram of force or kilopond, since it is nearly equal to those units.
dekan
a unit of angle measure equal to 10° or 1/36 circle. The ancient Egyptians divided the circle of the Zodiac into 36 divisions, which the Greeks called dekans. The unit is still used occasionally in astrology, where one dekan equals 1/3 sign.
dekare or decare
a metric unit of area equal to 10 ares, that is, 10 000 square meters or 0.1 hectare. In English units, the dekare equals approximately approximately 10,763.91 square feet, 1195.99 square yards, or 0.247 105 acres. Various traditional units of land area have been identified with the dekare, including the Middle Eastern dunum (see below), the Norwegian mål, the Greek stremma, and the Vietnamese cong.
dekatherm or decatherm (dtH)
a unit of energy equal to 10 therms, 1 million Btu, or about 1.055 057 gigajoules (GJ). This unit is used in the energy industry as a synonym for the million-Btu (MM Btu).
demal (D)
an obsolete unit of concentration in chemistry. From 1901 to 1964, the liter was officially defined to be exactly 1.000028 cubic decimeter. During this period, there was a small difference between measuring concentration in moles per liter and in moles per cubic decimeter. Concentration in moles per liter is called molar, while concentration in moles per cubic decimeter was called demal. This distinction has now disappeared along with the awkward definition of the liter. The former conversion was 1 demal = 1.000 028 molar.
demisemiquaver
a unit of relative time in music equal to 1/32 whole note or 1/64 breve.
denaro
a traditional Italian weight unit equal to 24 grani; its size varied from about 1.1 grams to 1.25 grams.
denier [1]
a traditional unit of yarn density. One denier is the density of a thread having a mass of 1 gram per 9 kilometers of length. The metric unit of yarn density is the tex; 1 denier equals 1/9 tex or 10/9 decitex.
denier [2]
a traditional French weight unit, comparable to the English pennyweight. Like the pennyweight, the denier is equal to 24 grains (1.275 grams). This unit was also called the scrupule.
dessertspoon or dessertspoonful (dsp)
a unit of volume sometimes used in food recipes. The dessertspoon is equal to 2 teaspoons; this is roughly equivalent to 10 milliliters in the U.S. and 7 milliliters in Britain.
dessiatina
a traditional unit of land area in Russia equal to 2400 square sadzhens. By coincidence, this makes the dessiatina very nearly the same as a hectare: it equals about 1.0925 hectare or 2.6996 acres.
deuce
an old English word for two, derived from the old French deus (now spelled deux). The word survives as the name for a two-spot showing in dice or a two card in card games. In tennis, "deuce" describes a tie situation in which a player must win the next two points in order to win the game.
diameter (dia)
a unit of magnification, equivalent to power. "Power" tends to be used in adjective phrases, such as "120-power eyepiece" or "7-power binoculars." "Diameters" often appear in declarative statements of magnification, such as, "the eyepiece lens magnifies 120 diameters."
diamond
a mysterious marking on many tape measures in the U.S. The diamonds mark a distance unit equal to exactly 8/5 feet (19.2 inches or 48.768 centimeters). This is useful to carpenters if they wish to place 5 studs, floor joists, etc., in a distance of 8 feet.
dicker
a traditional unit of quantity equal to 10. The word is an English version of the Latin word for ten, decem. After being used for centuries, especially by traders in furs and skins, the dicker has nearly disappeared from the English language.
Didot point
a unit of distance used in typography; see point [2].
dieb. alt., dieb. tert.
abbreviations for the Latin diebus alternis, every other day, and diebus tertius, every third day, units of frequency sometimes used in medical prescriptions.
digit
a historic unit of distance equal to the width of a person's finger. Used in all the ancient civilizations of the Middle East and Mediterranean, the digit was equal to 0.75 inch or 19 millimeters with only the smallest variations. Typically, there were 4 digits in a palm, 16 in a foot, and 24 (sometimes 28) in a cubit. The word digit is from the Latin word for a finger or toe, digitus.
dimension (dim)
a mathematical unit measuring the number of independent directions in a set or space. Traditionally, a space has as many dimensions as there are mutually perpendicular directions at each point in the space: thus a line has 1 dimensions, a plane has 2 dimensions, and the ordinary space we live in has 3 dimensions. The theory of relativity is set in a "space-time" having 4 dimensions, and higher dimensional spaces are frequently used in science and economics. In addition, mathematicians have developed several methods for assigning fractional dimensions to certain complex sets.
DIN
the initials of Deutsches Institut für Normung, the German standards agency. Outside the U.S., film was generally marked with a DIN speed rating such as DIN 24 (corresponding to ASA 200). A difference of 3 in the DIN rating corresponds to a doubling of the film speed; that is, DIN 27 film (ASA 400) is twice as fast as DIN 24. The ASA and DIN ratings are now combined, with a degree symbol on the DIN number, and marked with ISO (the initials of the International Standards Organization). Thus DIN 27 film is now marked ISO 400/27°.
diopter (dpt or D)
a metric unit used in optics to measure the refractive power of a lens. Each lens has a focal length, defined to be the distance from the center of the lens to the point at which the lens focuses light. The shorter the focal length, the greater the refractive power of the lens. The refractive power of the lens, in diopters, equals 1 divided by the focal length of the lens, in meters, so 1 diopter = 1 m-1. The unit is often spelled dioptre outside the United States. The lower case Greek letter delta is also used as a symbol for the diopter.
diraa
a traditional Egyptian unit of distance equal to about 58 centimeters (22.8 inches).
djerib
see jerib.
dk-
an incorrect symbol for the metric prefix deka- (10), sometimes seen in combinations such as dkg for the dekagram or dkm for the dekameter. The correct symbol is da-.
Dobson unit (DU)
a unit used in geophysics to measure the ozone in the atmosphere. One Dobson unit represents the amount of atmospheric ozone that would form a uniform layer 0.01 millimeter (10 micrometers) thick at standard temperature (0 °C) and pressure (1 atmosphere or 1013.25 millibars). The Dobson unit equals 10-5atmo- meter. Under normal conditions, the atmosphere contains about 300 Dobson units of ozone, but this falls to 100 Dobson units or less in the "ozone holes" over the Earth's poles. The unit is named for the British physicist G.M.B. Dobson; in 1920 he invented a spectrometer to measure ozone concentrations from the ground.
dog year
an informal unit of time equal to 1/7 of a normal, or "human" year. According to folklore, dogs age 7 times faster than humans.
dollar ($)
a unit used in nuclear engineering to describe the "reactivity" of a nuclear reactor. For a particular reactor, one dollar is the reactivity at which the chain reaction is just self-sustaining. The size of the unit varies with the design of the reactor; a typical size is approximately 10-5 = 0.001%. For a discussion of reactivity, see inhour.
donkey power
a somewhat light-hearted metric unit of power equal to 250 watts or about 0.3353 horsepower.
donum
see dunum (below).
doppelzentner (Dz)
a name often used in Germany for the decitonne or metric quintal. Since a zentner is 50 kilograms, a doppelzentner or "double" zentner equals 100 kilograms (about 220.462 pounds).
double
a traditional unit of volume for liquor equal to 2 shots. In the U.S., a double equals 2 fluid ounces or about 59.15 milliliters.
doublet
another name for a pair.
douzième
a traditional unit of distance used in watchmaking. The word is French for "twelfth," and a douzième is equal to 1/12 Swiss ligne. This is about 188 micrometers (microns) or 7.4 mils.
dozen (doz or dz)
a familiar unit of quantity equal to 12. Division into units of 12 rather than 10 has the advantage that 12 can be evenly divided into halves, thirds, or quarters. For this reason, units of 12 have been common since the earliest civilizations of the Middle East. "Dozen" comes from an old French word dozaine related to the Latin word duodecem, "twelve."
dpi
a unit used to measure the resolution, or sharpness, of a photograph or video image. The unit, an abbreviation for "dots per inch," remains in use even though individual picture elements are now called pixels rather than dots.
dpm, dps
abbreviations for decay per minute and decay per second, units used in the measurement of rates of radioactivity. 1 dps is properly called 1 becquerel (Bq). 1 dpm is equal to 1/60 becquerel or 0.45045 picocuries.
drachm
traditional British spelling of dram (see below).
drachma
a traditional unit of weight in Greece and the eastern Mediterranean. The drachma was a Greek coin, and the unit was originally the weight of the coin. This is about 3.2 grams, 49.3 grains, or 0.113 ounce avoirdupois. There is also a traditional Dutch weight unit of the same name, but it is somewhat larger: 3.906 grams, 60.28 grains, or 0.1374 ounce avoirdupois. Compared to a similar English unit, the Greek drachma is about 2.5 pennyweight and the Dutch is almost exactly 3 pennyweight.
drachme
a traditional unit of weight in France and Germany, generally between 3.7 and 3.9 grams.
dram (dr) [1]
a unit of weight in the traditional avoirdupois system (see pound [1]), equal to 1/16 ounce or 1/256 pound. One dram equals about 1.7718 gram. The word dram comes from a Latin weight unit, the dragma, and derives ultimately from the Greek drachme, meaning a handful. The word is usually spelt "drachm" in Britain and "dram" in the United States, but both spellings are pronounced "dram."
dram (dr) [2]
a unit of weight in the traditional system of English apothecaries (see scruple), equal to 60 grains, 3 scruples, 1/8 troy ounce, or approximately 3.8879 gram. The apothecaries' dram is sometimes abbreviated dr ap to distinguish it from the avoirdupois dram. It is equivalent to about 2.1943 avoirdupois drams. There is a similar Italian unit, the dramma, equal to 72 grani or roughly 3.5 grams.
dram (dr) [3]
a traditional unit of volume. See fluid dram.
drex
a unit traditionally used in the textile industry to measure the density of a single fiber of yarn. One drex equals a density of one gram per 10 kilometers of length, or 1 microgram per centimeter (µg/cm). Since 1 drex equals 0.1 tex, the unit is now called the decitex.
drill sizes
traditional drill sizes are numbers 1-80, with larger numbers indicating smaller drills. Number 1 has diameter 0.2280 inch and number 80 has diameter 0.0135 inch. Larger sizes are designated by letters or by specifying the diameter directly in 64ths of an inch. The metric drill size is the diameter in millimeters.
drink
a unit measuring the alcohol content of beverages, used in describing the medical effects of alcohol. U.S. physicians generally consider one drink equal to 0.5 U.S. fluid ounces of alcohol; the appropriate metric equivalent would be 15 milliliters. In U.S. fluid units, one drink corresponds to about 4 ounces of wine or 1.25 ounces of whiskey.
drop (gtt) [1]
a unit of volume used in pharmacy. Traditionally the drop was another name for a minim, a unit of volume equal to 1/60 fluid dram or 1/480 fluid ounce (about 0.0616 milliliter in the U.S., 0.0592 milliliter in Britain). Now that prescriptions are written in metric units, the pharmacist's drop is equal to exactly 0.05 milliliter. The traditional abbreviation is from the Latin gutta, drop. Originally, gt was the symbol for a single drop, with gtt being the plural.
drop [2]
a informal unit of volume used in recipes. According to some older kitchen references, 24 drops = 1/4 teaspoon; with U.S. definitions this makes the drop equal to 1/576 fluid ounce or about 0.051 milliliter, comparable to the pharmacist's drop (previous entry).
drop [3]
a traditional unit of weight in Scotland, equal to 1/16 troy ounce or about 1.944 grams.
drum
a unit of volume sometimes used in the oil trade. A standard drum of oil contains 55 U.S. gallons or about 208.198 liters. One drum equals 1.3095 barrels [2].
dry pint, dry quart
common names for the U.S. pint and quart units used for dry commodities, used to distinguish these units from the pint and quart used for liquids. The dry units are equal to about 1.163647 times the corresponding liquid units.
dry ton
see wet ton.
duet or duo
a traditional unit of quantity equal to 2. The word duet is a French version of the Italian duo, two.
dunum or donum
a traditional unit of land area in the Middle East. The unit is of Turkish origin, but it seems to be obsolete in modern Turkey. As it is commonly used in Israel and Jordan, the dunum is a metric unit equal to 1000 square meters or 0.1 hectare (about 0.2471 acre).
duro, durometer
a measure of the hardness of plastic, rubber, or similar material, made an instrument called a durometer. These instruments test the material with a steel pin, and the depth of the indentation made by the pin is read by a calibrated spring meter. There are various versions of the durometer, designed for materials of different kinds; to be called durometers, all versions must conform to standards set by standards organizations such as the American Society for Testing and Materials (ASTM). Durometer scales run from 0 to 100, with larger numbers indicating harder material. The different scales are identified by letters, including A, B, C, D, O, DO, and OO. However, since only one type is used for a particular material, the scale designation is often omitted. Typical notations used are "50 duro", "65 durometer A", and "D40 duro".
dwt
abbreviation for deadweight ton (see above); also the traditional symbol for pennyweight.
dyad
a unit of quantity equal to 2. This unit is frequently used in cell biology and biotechnology.
dyne (dyn)
the CGS unit of force. One dyne is the force that accelerates a mass of one gram at the rate of one centimeter per second per second. Expressed in SI units, the dyne equals 10-5newton. This is quite a small force: it equals about 2.248 x 10-6 pound of force (lbf) in the traditional English system. The word dyne comes from the Greek dynamis, power. See Force