C

c: a symbol for the velocity (or "speed") of light. One of the fundamental principles of physics is that light always travels at the same velocity in a vacuum, exactly 299, 792,458 meters per second or about 670,617,300 miles per hour. Another fundamental principle is that nothing can travel faster than light. At velocities which are large fractions of the speed of light, the theory of relativity predicts a variety of strange physical effects. In calculations involving relativity, speeds are customarily expressed as fractions of the speed of light, such as 0.95 c.
C [1]: the Roman numeral 100, sometimes used as a unit of quantity or as a prefix meaning 100, as in Cwt (hundredweight) or CCF (100 cubic feet).
C [2]: a symbol for international standard paper sizes, followed by the size number, as in C4. The C series of sizes is used primarily for envelopes.
C [3]: a unit of relative current for batteries. For a particular battery, a current of 1C is a current in amperes numerically equal to the rated capacity of the battery in ampere hours. In other words, a 1C current will completely charge or discharge the battery in one hour.
cabellaria: a traditional unit of land area in Spanish speaking countries. In Spain and Peru the cabellaria is equal to 60 fanegas, which is roughly 40 hectares (100 acres). In Central America it equals 60 manzanas, which is roughly 45 hectares (110 acres).
cable: a unit of distance formerly used at sea. The traditional mariner's cable was 120 fathoms long. This is equal to 720 feet, or 0.1185 nautical mile, or about 219.4 meters. A different version of the cable, used in the British Navy, was equal to exactly 0.1 nautical (Admiralty) mile, which is 608 feet or about 185.3 meters.
calendar year: a civil unit of time, equal to 365 days or (in leap years) 366 days. See year [2].
caliber (cal) [1]: a unit used to express the diameter of the bore of a gun. (The bore is the inside diameter of the gun barrel.) Traditionally, the diameter was stated in inches, so that ".22 caliber" referred to a pistol having a bore of 0.22 inches (5.588 mm). This usage is declining, because bore diameters of many guns are now stated directly in millimeters. "Caliber" is the American spelling; elsewhere the unit is often spelled "calibre."
caliber (cal) [2]: a measure of the relative length of a gun barrel, defined as the length divided by the diameter of the bore. Thus a "50-caliber" gun on a warship has a barrel 50 times longer than its bore. Confining the shell within the barrel for a longer time increases the velocity, so guns with a higher caliber usually have a longer range.
Callipic cycle: a unit of time equal to 76 years or 4 Metonic cycles, formerly used in astronomy in predicting the phases of the Moon. After the passage of one Callipic cycle, the phases of the Moon repeat essentially on the same calendar dates as in the preceding cycle. The cycle is named for the Greek astronomer Callipus, who discovered it in 330 BC.
calorie (cal): the CGS unit of heat energy. This calorie (also called a gram calorie or small calorie) is the amount of heat required at a pressure of one atmosphere to raise the temperature of one gram of water by one degree Celsius. Unfortunately, this varies with the temperature of the water, so it is necessary to specify which degree Celsius is meant. A traditional choice was the degree from 14.5°C to 15.5°C; raising the temperature of water through this range requires 4.1858 joules, a quantity called the 15° calorie. Another choice produces the thermochemical calorie, equal to exactly 4.184 joules. More common today is the international steam table calorie, or IT calorie for short, defined by an international conference in 1956 to equal exactly 4.1868 joule or about 0.00396832 British thermal units (Btu). The name of the unit comes from the Latin calor, heat.
Calorie (Cal or kcal): the MKS unit of heat energy (also called the kilogram calorie or large calorie). This unit is often (but not always!) distinguished from the small calorie by capitalizing its name and symbol. The large calorie is the amount of heat required at a pressure of one atmosphere to raise the temperature of one kilogram of water by one degree Celsius. Since this is 1000 times as much water as mentioned in the definition of the small calorie, the large calorie equals 1000 small calories, 4.1868 kilojoules, or 3.9683 Btu. These are the calories that joggers are trying to get rid of, the ones we gain by eating. The use of the same term "calorie" for two different-size units is endlessly confusing, but we seem to be stuck with it.
cana, canna, canne: traditional units of distance in Spain, Italy, and southern France, respectively. The cana varied in size, but it was most often defined as 8 palmos, which makes it the Mediterranean version of the fathom, equal to roughly 2 meters (6.5 feet). In Italy a measuring stick is still called a canna metrica. The unit is sometimes translated "rod" in English, but "fathom" is the proper choice.
candela (cd): the fundamental SI unit for measuring the intensity of light. Candela is the Latin word for "candle." The unit has a long and complicated history. Originally, it represented the intensity of an actual candle, assumed to be burning whale tallow at a specified rate in grains per hour. Later this definition was replaced with a definition in terms of the light produced by the filament of an incandescent light bulb. Still later a standard was adopted which defined the candela as the intensity of 1/600,000 square meter of a "black body" (a perfect radiator of energy) at the temperature of freezing platinum (2042 K) and a pressure of 1 atmosphere. This definition has also been discarded, and the candela is now defined to be the luminous intensity of a light source producing single-frequency light at a frequency of 540 terahertz (THz) with a power of 1/683 watt per steradian, or 18.3988 milliwatts over a complete sphere centered at the light source. The frequency of 540 THz corresponds to a wave length of approximately 555.17 nanometers (nm). Light of this frequency has a yellow-green color and roughly the same visual brightness as ordinary daylight. In addition, normal human eyes are more sensitive to this wavelength than to any other. In order to produce 1 candela of single-frequency light of wavelength l, a lamp would have to radiate 1/(683V(l)) watts per steradian, where V( l) is the relative sensitivity of the eye at wavelength l. Values of V(l) are defined by the International Commission on Illumination (CIE).
candle (cd): an older name of the unit now known officially as the candela (see above), or for the candlepower (see below).
candlepower (cp): a unit formerly used for measuring the light radiating capacity of a lamp or other light source. One candlepower represents the radiating capacity of a light with the intensity of one "international candle," or about 0.981 candela as now defined. Since 1948 the candela has been the official SI unit of light intensity, and the term "candlepower" is no longer used much as a unit of measurement. Instead, "candlepower" often means a measurement of light intensity in candelas.
candy: a traditional weight unit of South Asia. The candy was quite variable, generally within the range 500 to 800 pounds (225 to 365 kilograms). In the international cotton trade, the candy was generally equal to exactly 7 (British) hundredweight, which is 784 pounds or 355.62 kilograms.
can sizes: Food cans are identified by their nominal dimensions, diameter × height. (The "nominal" dimensions are somewhat larger than the actual dimensions, as is the case for lumber and some other products.) In the metric world the dimensions are 2- or 3-digit numbers representing dimensions in millimeters. In traditional U.S. nomenclature, the dimensions are stated as 3-digit numbers, with the first digit representing inches and the remaining two digits representing 16ths of an inch. A common can for fruits and vegetables, for example, is designated 83×116 in metric terminology, or 307×409 (3-7/16×4-9/16) in traditional terminology. If only one number is mentioned it is the diameter; thus a "404" can has a nominal diameter of 4-4/16 = 4.25 inches and a "65" can has a nominal diameter of 65 millimeters.
cantar: an English spelling for the Arab form of the quintal. In recent years, the cantar has been interpreted as a metric unit equal to 50 kilograms (110.23 pounds); traditional cantars tended to be a few percent larger than this.
Cape foot (cf): a traditional unit of distance in South Africa. The Cape foot equals 12.396 English inches, 1.0330 English foot, or 31.4858 centimeters. This unit is not the traditional Dutch foot, but it is similar in length to the "Rhine foot" of northern Germany. The Cape foot was widely used for land measurement and appears on many deeds in South Africa. Europeans often referred to South Africa as "The Cape," meaning the Cape of Good Hope.
Cape rood: a traditional unit of distance in South Africa, equal to 12 Cape feet or 12.396 English feet (3.7783 meters). See also rood.
carat (ct or c) [1]: a unit of mass used for diamonds and other precious stones. Some say the word carat comes from the Arabic qirrat, meaning the seed of a coral tree, and some say it comes from the Greek keration, a carob bean. Perhaps both seeds were used as standards for weighing precious stones. Traditionally the carat was equal to 4 grains. The definition of the grain differed from one country to another, but typically it was about 50 milligrams and thus the carat was about 200 milligrams. In the U. S. and Britian, the diamond carat was formerly defined by law to be 3.2 troy grains, which is about 207 milligrams. Jewelers everywhere now use a metric carat defined in 1907 to be exactly 200 milligrams.
carat (ct or c) [2]: in Britain, the spelling "carat" is used for the unit of gold purity known in America as the karat.
carga: a traditional unit in Spanish and Portuguese speaking countries. The word mean "load". It was often used as a unit of mass or weight equal to 3 quintals or as a unit of volume equal to the volume holding 3 quintals of the commodity being shipped.
Carnegie unit: a unit of academic credit used in college admissions decisions in the U.S. The unit was introduced by the Carnegie Foundation for the Advancement of Teaching in 1914 to provide colleges with a standard measure of students' course work in high schools. A Carnegie unit represents the equivalent of one academic year of study in a subject in a class meeting 4 or 5 times a week for 40 to 60 minutes per meeting, a minimum of 120 hours of total class time.
case: a conventional unit of sales for many items, varying with the item and over time. A case of wine, for example, is traditionally twelve 750- milliliter bottles. The word comes from the Latin capsa, a chest.
category: the ranking of a hurricane on the Saffir-Simpson scale, used by the U.S. National Weather Service. A somewhat different scale of categories is used for tropical cyclones by the Australian Bureau of Meterology.
catty: a weight unit of the colonial period in East and Southeast Asia, originating as the kati in Malaya. The catty varied a little from market to market. Typically it was equal to about 1 1/3 pounds avoirdupois (604.79 grams), and it is still equal to that weight in Malaysia. In Thailand, the catty is used now as a metric unit equal to exactly 600 grams (1.3228 pounds). In China, the catty was identified with the jin, a traditional Chinese unit.
cb-: abbreviation for "cubic," seen in combinations such as cbm (cubic meter) or cbft (cubic foot). The proper symbol for cubic meters is now m³, not cbm.
cc: an alternate symbol for the cubic centimeter (cm³). This symbol is obsolete and should not be used; cm³ should be used in its place. The cubic centimeter is the same volume as the milliliter (mL).
CCF: an abbreviation for 100 cubic feet. Local water and sewer utilities often sell water in CCF units; for this purpose one CCF equals about 748.05 gallons (U.S.) or about 2831.7 liters. Utilities sometimes sell natural gas in CCF units; for this purpose the CCF is really a unit of energy equivalent (roughly) to the threm.
CE [1]: abbreviation for "common era." This abbreviation is a non-religious designation used in place of the traditional AD for years of the common or Christian era. Years of the common era are supposed to be counted from the birth of Jesus of Nazareth, founder of the Christian religion. However, the year-numbering system was not established until more than 500 years later. It is based on calculations of the monk Dionysius Exiguus placing Jesus's birth in the Roman year 753 AUC. This calculation is definitely wrong; the Biblical account of Jesus's birth implies that he was born several years before the death of Herod the Great, who died, we now know, in 4 BC. Thus the calculation of the common era is off by 6 or 7 years at least, and the third millennium actually started several years ago! In the conventional use of the common era system, there is no year 0 and the year prior to 1 CE is designated 1 BCE. In astronomy, however, it simplifies calculations to define the year 0 CE = 1 BC and to apply negative numbers to earlier years. Thus Herod died in -3 CE, and, in general, -n CE is the year more commonly called n + 1 BC. See Time
CE [2]: French abbreviation for colonne d'eau, water column, seen in pressure measurements. See centimeter of water (below) or millimeter of water.
Celsius: see degree Celsius. The word "degree" is often omitted in informal statements of temperature, as in "we expect a high of about 23 Celsius today". See Converter.
Celsius heat unit (Chu): a unit of heat energy, equal to the energy required to raise the temperature of one pound of water by 1°C at standard atmospheric pressure. 1 Chu is equal to exactly 1.8 Btu, approximately 453.59 IT calories (see above), or 1.8991 kilojoules. The unit is also called the centigrade heat unit.
cent [1]: an old English unit of quantity, usually equal to 100 but sometimes 120 (the great or long hundred) or some other figure of similar size.
cent [2]: an informal name for 1/100 of almost any unit, for example, a centiliter (0.01 liter).
cent [3]: a unit used in music when it is necessary to specify the ratio in frequency between two tones with great precision. There are 100 cents in a semitone, or 1200 in an octave. If two notes differ by 1 cent, the ratio between their frequencies is 2^1/1200 or approximately 1.0005778.
cent [4]: a unit used in nuclear engineering to describe the "reactivity" of a nuclear reactor, equal to 0.01 dollar. For a discussion of reactivity, see inhour.
cental (cH or ctl): an alternate name in Britain for the U.S. hundredweight, which is equal to exactly 100 pounds (the British hundredweight is 112 pounds). Introduced by British merchants around 1850, the name was apparently coined after the model of the quintal. The cental has sometimes been confused with the centner (see below).
centi- (c-): a metric prefix meaning one hundredth, or 0.01. The prefix comes from the Latin word centum for one hundred. See Prefix
centiare (ca): a metric unit of area. The centiare equals 0.01 are, which is exactly 1 square meter (about 10.7639 square feet).
centibar (cbar or cb): a metric unit of pressure identical with the kilopascal (kPa). One centibar equals 0.01 bar, 7.5006 torr, or 0.1450 pounds per square inch (lbf/in² or psi). The centibar is traditionally used in agriculture as a unit of soil water tension (the water pressure on the roots of plants) as measured by devices called tensiometers.
centigrade: a temperature scale; see degree centigrade.
centigram (cg): a metric unit of mass equal to 10 milligrams or about 0.154 grain.
centihg: an informal unit of pressure equal to 1 centimeter of mercury. This is equivalent to 10 millimeters of mercury, approximately 0.3937 inHg, 0.1933 lb/in², 13.33 millibars, or 1333 pascals. The word is pronounced "sentig".
centiliter (cl or cL): a common metric unit of volume. One centiliter equals 10 cubic centimeters; this is about 0.61024 cubic inch, 0.3318 U.S.fluid ounce or 0.3519 British fluid ounce. In the kitchen, a centiliter is roughly equal to 2 U.S. teaspoons (or 0.704 British tablespoonful).
centimeter (cm) [1]: the fundamental unit of distance in the CGS metric system, equal to 0.01 meter. One centimeter is about 0.393 700 787 inch.
centimeter (cm) [2]: an obsolete name for the statfarad (approximately 1.11 picofarad), the CGS electrostatic unit of capacitance.
centimeter (cm) [3]: an obsolete name for the abhenry, the CGS electromagnetic unit of inductance. The abhenry is the same as the nano henry.
centimeter of mercury (cmHg): a traditional unit of pressure equal to 10 mmHg, 1.333 22 kilo pascal, or about 0.193 pounds per square inch.
centimeter of water (cmH₂O, cm WC, cm CE, cm WS): a unit of pressure equal to the pressure exerted at the Earth's surface by a water column (WC) 1 centimeter high. This is about 98.067 pascals, 0.98067 millibars, 0.3937 inch of water, or 2.04 pounds per square foot. The unit is used in respiratory medicine and elsewhere to measure air pressures. The French symbol is cm CE (colonne d'eau), and the German symbol is cm WS (Wassersäule).; a unit of genetic separation used in genetics and biotechnology. If two locations on a chromosome have a 1% probability of being separated during recombination in a single generation, then the distance between those locations approximately equal to one million base pairs. The unit honors the pioneering American geneticist Thomas Hunt Morgan (1866-1945), who received the 1933 Nobel Prize in Medicine for his discoveries concerning the role played by the chromosome in heredity.
centinewton (cN): a metric unit of force equal to 0.01 newton. This unit has some popularity in engineering as a substitute for the gram of force (gf), since it equals about 1.01972 gf (about 0.0360 ounces of force in the English system). In the textile industry, the breaking strength of fibers is commonly expressed in centinewtons per tex.
centipoise (cP, cPs, or cPo): a common metric unit of dynamic viscosity equal to 0.01 poise or 1 millipascal second (mPa·s). The dynamic viscosity of water at 20 °C (68 °F) is about 1 centipoise. The correct symbol for the unit is cP, but cPs, cPo, and even cps are sometimes used.
centistokes (cSt): a common metric unit of kinematic viscosity equal to 0.01 stokes or 1 mm²/s. The viscosity of lubricating oils and many other liquids is frequently stated in centistokes. Although the centistokes is not an SI unit it is likely to remain in use, since it provides a convenient and traditional name for an SI-appropriate quantity (1 mm² /s). The older symbol cS should not be used for this unit, since S is now the symbol for the siemens.
centner [1]: the English name for a German weight or mass unit, the zentner, equal to 50 kilograms or about 110.231 pounds. The name centner should not be used for the cental (see above).
centner [2]: a Russian weight or mass unit equal to 100 kilograms (approximately 220.4623 pounds). This centner, also used in Ukraine and the other former Soviet republics, is equal to the decitonne and to the metric quintal; it is twice the size of the centner [1] used in western Europe.
centrad: a unit of angle measure equal to 0.01 radian or about 0.572958° (34' 22.65"): a "centiradian," in other words.
century [1]: a unit of quantity equal to 100. In ancient Rome, a "century" was originally a company of about 100 soldiers led by an officer called a centurian.
century [2]: a traditional unit of time equal to 100 years. In naming centuries, historians recall that there was no year 0 in the conventional year numbering system. Thus the First Century included the years 1-100 and the Twentieth Century includes the years 1901-2000. (As an example in the other direction, the Fifth Century BC includes the years 500-401 BC.) However, there is strong popular sentiment for considering the Twentieth Century to be the years 1900-1999, making 2000 the first year of the Twenty-first Century.
cetane number: a measure of the ability of diesel fuel to reduce engine knocking. The cetane number plays the same role in diesel engine technology that the octane number plays in conventional automobile engine technology. It is the percentage by volume of cetane which must be added to methylnaphthalene to give the mixture the same resistance to knocking as the diesel fuel sample being tested.
cfm, cfs: traditional abbreviations for cubic feet per minute and cubic feet per second, respectively. 1 cfm = 28.317 liters per minute and 1 cfs = 28.317 liters per second.
CFU: abbreviation for "colony forming units," a count of the number of active bacterial cells in preparations of Lactobacillus acidophilus and other "friendly" organisms of the digestive system. Counts as high as one billion CFU per gram are not uncommon.
chain (ch): a unit of distance formerly used by surveyors. The traditional British surveyor's chain, also called Gunter's chain because it was introduced by the English mathematician Edmund Gunter (1581-1626) in 1620, is 4 rods long: that's equal to exactly 1/10 furlong, 22 yards, or 66 feet (20.1168 meters). Gunter's chain has the useful property that an acre is exactly 10 square chains. The chain was divided into 100 links. American surveyors sometimes used a longer chain of 100 feet, known as the engineer's chain or Ramsden's chain. (However, Gunter's chain is also used in the U.S.; in fact, it is an important unit in the Public Lands Survey System.)
chalder or chaldron (chd): a traditional British unit of volume or weight, used for dry commodities such as coal or lime. As a volume measure, the chaldron is equal to 36 bushels, or 288 British Imperial gallons; this is equivalent to 46.237 cubic feet or 1.3091 cubic meters. As a measure for coal, the chalder equals 1/8 keel or 53 hundredweight (5936 pounds or 2692.52 kilograms). The words "chalder," "chaldron," and "cauldron" are English spellings of the same old French word, which originally meant a large kettle.
champagne quart: see quart [2].
character (char): a unit of information used in computer science and telecommunications. One character is usually equal to 8 bits or one byte. There is, however, a new coding system designed to accommodate all the characters used in all the world's languages; this system, Unicode, assigns 16 bits or 2 bytes to each character code.
charka: a traditional Russian unit of volume containing about 123.0 milliliters, 4.159 U.S. fluid ounces or 4.329 imperial fluid ounces. There are 6.25 charki in a boutylka (bottle) and 10 in a schtoff. The word charka means a cup or glass.
Charrière, Charrière gauge (Ch): a unit of distance used for measuring the diameters of small tubes such as catheters, fiber optic bundles, etc. The gauge number is the diameter of the tube in units of 1/3 millimeter. In English-speaking countries, the scale is usually called the French gauge and the unit is simply called "French." Charrière was a nineteenth century French instrument manufacturer.
cheval vapeur (cv): French name for the metric horsepower.
ch'ih: a unit of distance used in China during the colonial period. The ch'ih equals 10 t'sun, 35.814 centimeters, or 14.1 inches. There are 1800 ch'ih in a li.
chiliad [1]: a unit of quantity equal to 1000. The word comes from the Greek numeral 1000, chilioi, which is also the origin of the metric prefix kilo-. Pronounced "killiad," the chiliad was once fairly common in learned writing, but it has nearly disppeared from use today.
chiliad [2]: another name for a millennium (1000 years).
chilo-: Italian spelling of the metric prefix kilo- (1000).
chin: one of several spellings in English for the jin, a traditional Chinese weight unit.
chopine: a traditional French unit of volume. The unit varied regionally, but by the 18th century it was more or less standardized as 23.475 cubic pouces (465.7 milliliters). The chopine is obsolete in France today, but the word survives (especially in French Canada) as a French name for the English pint units.
choppin: a traditional Scottish unit of volume equal to 2 mutchkins or 1/2 Scots pint. The choppin is equivalent to about 51.702 cubic inches, 0.8953 U.S. liquid quart, 1.491 British imperial pints, or 847.3 milliliters.
cicero: a unit of distance used by typesetters and printers in continental Europe, equal to 12 Didot points. This is approximately 0.1780 inch or 4.52 millimeters. The cicero corresponds to the British and American pica. See typographic
cinque: an old English word for the number 5, pronounced "sink" and derived from the French number 5, cinq. In English history, the original Cinque Ports were Sandwich, Dover, Hythe, Romney and Hastings. The word survives today as the name for a 5-spot showing in dice, or for a 5-card in card games.
circle (cir): the traditional unit of angle measure, divided into 2 piradians or 360 degrees.
circular inch, circular mil: informal units of area. A circular inch [mil] is the area of a circle one inch [mil] in diameter. Since the area of a circle is proportional to the square of the diameter, a circle of diameter X inches [mils] has an area of X² circular inches [mils]. The circular inch is equal to approximately 0.785398 square inches or 5.06707 square centimeters. The circular mil is equal to approximately 0.785 398 square mils or 506.707 square micrometers.
Clark degree: see degree [4].
clausius: a unit of entropy. Entropy is a measure of the extent to which heat or energy in a physical system is not available for performing work. It is computed in units of energy per kelvin. One clausius is equal to 1 Calorie per kelvin (Cal/K) or 4.1868 kilojoules per kelvin (kJ/K). The unit honors the German physicist Rudolf Clausius (1822-1888), who introduced and named the concept of entropy in 1850.
click: U.S. military slang for the kilometer (about 0.621 mile). Also spelled klick or klik. Widely used during the Vietnam War, this unit seems to have been invented by U.S. troops in Germany during the 1950s. Occasionally it was used as a non-metric unit equal to 1000 yards (0.9144 kilometer).
clo: a unit of thermal insulation used for clothing. One clo is intended to represent the insulation required to keep a resting person warm in an indoor room at 70 °F (21.1°C). The rate of a person's heat loss is measured in watts per square meter of skin area per kelvin of temperature difference across the clothing; the value of insulation is measured by the reciprocal of this rate, in square meter kelvins per watt (m² K/W). One clo is equal to 0.155 m²K/W or 1.550 togs.
clove: an old English unit of weight. A clove is usually considered equal to 1/2 stone; that's 7 pounds (3.175 kilograms) by the modern definition of the stone, but in the past the clove varied from 6.5 to 8 pounds.
clusec: a unit of power used to express the performance or leakage of vacuum pumps. One clusec represents a flow of 10 milliliters per second at a pressure of one micrometer (or micron) of mercury. This is equivalent to 0.01 lusec or 1.333 micro watts. The name of the unit is an acronym for "centi-lusec."
coffee measure: a flat-bottomed scoop or spoon used to measure coffee beans for grinding in U.S. homes. The coffee measure holds 2 U.S. tablespoons (about 29.57 milliliters).
coffeespoon: a unit of volume formerly used in U.S. food recipes. A coffeespoon is 1/2 teaspoon, 1/12 fluid ounce, or about 2.5 milliliters.
collothun: an ancient Persian unit of liquid volume, equal to 1/8 artaba or (in recent centuries) about 8.25 liters.
color units: several systems have been devised to measure colors. The subject was mostly academic until recently, but now computers require precise methods for describing the colors to be displayed or printed. Computer monitors use the RGB system, which specifies colors with three variables measuring the intensity of the three primary colors red, blue, and green in the color. Each variable is specified by one byte and therefore takes values in the range 0 to 255. If all three are 0, the resulting color is black; if all three are 255 the resulting color is white. The RGB settings for the Green background of this page are R=170, G=255, and B=221. Since it is difficult to estimate the relative amounts of red, green, and blue needed to create a particular color, many graphics design programs use the HSV color system, which describes colors using three variables called hue, saturation, and value. Once again, all three variables are assigned values from 0 to 255. Hue, which is what we call "color" in ordinary language, is described on a circular scale. Hue values begin with red at 0 and run through yellow, green, blue, and purple before returning to red at 255. Saturation is the purity of the color, the extent to which it is not watered down with gray. The pure color has saturation 255. As saturation is reduced, the color becomes grayer, until at saturation 0 the color is replaced by a neutral gray of the same intensity as the original color. The value (or intensity) of the color is its brightness. The pure or most natural form of the color has value in the middle of the scale, at 127. As value is increased the color becomes brighter, becoming pure white at value 255. In the opposite direction, the color becomes less bright, becoming black at value 0. This page's background has hue 113, saturation 240, and value 200 in the HSV system. See Color
colpa, colp, or collop: a traditional Irish unit. The colpa was originally a unit of livestock equal to one cow or horse or to 6 sheep. Later it was used as a unit of pastureland equal to the pasturage supporting one colpa of livestock. This varied according to the quality of the land, but it was roughly equal to the Irish acre (0.6555 hectare). "Collop" is an English version of the Irish word "colpa."
column inch (col in): a unit of relative area used in journalism. A column inch is an area one column wide and one inch deep. The width of a column varies; a standard size in the U.S. is 2-1/16 inch. At this width a column inch is 2.0625 square inches or about 13.31 square centimeters.
cooling degree day (Cdd): see degree day.
cong: a metric unit of area used in Vietnam. One cong equals 1000 square meters, which is 0.1 hectare, 0.24177 acres, or 1196.00 square yards.
congius: a historic unit of liquid volume. The Roman congius was equal to about 3.2 liters (3.4 U.S. quarts or 2.8 British Imperial quarts); it was divided into 6 sextarii (sixths) which corresponded closely to modern pints. In the nineteenth century, the congius was used in British medicine and pharmacology as a name for the British Imperial gallon (4.54609 liters).
coomb or coom: a traditional British unit of volume used mostly for dry commodities. A coomb is 4 imperial bushels; this is equivalent to 5.1374 cubic feet or about 145.48 liters.
COP: an abbreviation for coefficient of performance, a measure of the efficiency of heat pumps, air conditioners, refrigerators, and freezers. The COP is the ratio of the useful energy output of the system divided by the electric energy input when the unit is operating in a steady-state test condition. Typical values are in the range 2-4. (The energy output exceeds the input, because the system takes advantage of the heat released or absorbed by the refrigerant when it condenses or evaporates.) A heat pump which delivers two units of cooling for each unit of electricity also rejects three units of heat; thus it has a COP of 2.0 for cooling or 3.0 for heat. For air conditioners, the COP is considered to equal the energy efficiency ratio (EER) divided by 3.412.
cord (cd) [1]: a traditional unit of volume used to measure stacked firewood. Like most traditional units of trade, the cord has varied somewhat according to local custom. In the United States, the cord is defined legally as the volume of a stack of firewood 4 feet wide, 8 feet long, and 4 feet high. (In Maryland, the law specifies that the wood be stacked "tight enough that a chipmunk cannot run through it." Presumably it is up to the buyer to provide the chipmunk.) One cord is a volume of 128 cubic feet, about 3.6247 cubic meters, or 3.6247 steres. The name apparently comes from an old method of measuring a stack of firewood using a cord or string.
cord (cd) [2]: in the U.S. timber industry, the cord is also used as a unit of weight for pulpwood. The weight varies with tree species, ranging from about 5200 pounds (2.36 metric tons) for pine to about 5,800 pounds (2.63 metric tons) for hardwood.
cord foot (cd ft): a traditional unit of volume used to measure stacked firewood. A cord foot is the volume of a stack of firewood 4 feet wide, 1 foot long, and 4 feet high. Thus the cord foot is 1/8 cord, or 16 cubic feet, or about 0.4531 cubic meter.
coulomb (C): the SI unit of electric charge. One coulomb is the amount of charge accumulated in one second by a current of one ampere. Electricity is actually a flow of particles called electrons, and one coulomb represents the charge on approximately 6.241506 x 10¹⁸ electrons. The coulomb is named for a French physicist, Charles-Augustin de Coulomb (1736-1806), who was the first to measure accurately the forces exerted between electric charges.
count (ct) [1]: a unit of quantity equal to 1. This unit is used in commerce to specify that the quantity stated represents a reliable count. For example, a carton marked "oranges 24 ct" contains exactly 24 oranges.
count (ct) [2]: a traditional unit measuring the texture of a fabric, equal to the number of threads per inch. A 100 count fabric has 39.37 threads per centimeter.
count (ct) [3]: an informal unit of volume in bartending. Bartenders usually fit bottles with pourers designed to restrict the flow to 0.5 fluid ounce per second (14.8 mL/s). They can then measure a quantity of liquid by counting, "one thousand one, one thousand two, ..." while pouring. This is much faster than using a measuring glass, and just about as accurate.
cousins: English, like most languages, has a procedure for stating the precise relationship between persons of common descent; a typical designation is "second cousins, once removed." First cousins are persons sharing a common grandparent; second cousins are persons sharing a common great-grandparent, and, generally, for n > 1, n-th cousins are persons sharing a common (n - 1)-times-great-grandparent; that is, for n-th cousins there are n + 1 generations in each of their descents from a common ancestor. The "removed" phrase is used when the number of generations in the descent from a common ancestor is not the same for both cousins: "r times removed" means the difference in the number of generations is r. Thus, for n-th cousins r times removed, one cousin has an (n - 1)-times-great-grandparent who is also an (n + r - 1)-times-great-grandparent of the other cousin (in the case of first cousins r times removed, one cousin has a grandparent who is also an (r - 1)-times-great-grandparent of the other cousin). For n-th cousins r times removed, there are n + 1 generations in the descent from a common ancestor for one cousin, and n + r + 1 generations for the other. The number of degrees of consanguinity between n-th cousins r times removed is 2n + r + 2.
covado, covido: Portuguese and Arabic names, respectively, for the cubit (see below). The Portuguese covado is equal to 3 palmos (66 centimeters, or 20.12 inches), while the Arabic covido is about 48 centimeters or 19 inches.
cps: a traditional abbreviation for cycles per second; also an incorrect symbol for the centipoise (see above).
cran: a traditional unit of volume formerly used by fishermen. The cran, originally intended to represent the volume of fish held by a barrel, was standardized at 37.5 Imperial gallons, or about 170.46 liters. This is somewhat larger than the usual Imperial barrel.
crannock: an old English unit of volume for grains. Never standardized, the crannock was roughly 10 bushels (350 liters).
crith: a unit of mass sometimes used in the physics and chemistry of gases. The crith is equal to the mass of a liter of hydrogen at standard temperature (0.01°C) and pressure (1 atmosphere); this is about 89.885 milligrams. The name comes from an ancient Greek word for a barleycorn.
crore: a traditional unit of quantity in India, equal to 10⁷ or 10 million. Large numbers are usually described in India using the crore and the lakh (10⁵); for example, the number 25,600,000 is called 2 crore 56 lakh and written "2,56,00,000".
crotchet: a unit of relative time in music equal to 1/4 whole note or 1/8 breve. The word, pronounced crotch-it, comes from the old Norse word krok for a hook; in this context it refers to the traditional hooked symbol for a quarter note.
cu: abbreviation for "cubic," as in cubic feet or cubic meters. This symbol is forbidden by the SI, but it remains in common use in ordinary English text. The proper symbol for cubic feet is now ft³, not cu. ft.
cuadra [1]: a traditional Latin American unit of distance. The cuadra is generally equal to 100 varas (about 84 meters or 275 feet) in Central America and northern South America. In Argentina and Chile, the cuadra was formerly equal to 150 varas (roughly 130 meters or 410 feet).
cuadra [2]: a traditional Latin American unit of area equal to one square cuadra [1]. Except in Argentina and Chile, this is 10 000 square varas, generally in the range 1.75-1.85 acres (0.71-0.75 hectare). In Argentina and Chile, the cuadra was 22 500 square varas (4.18 acres or 1.69 hectare).
cuartillo: a traditional Spanish unit of volume comparable to the liter or the English quart. The cuartillo equals 4 octavillos or 1/4 almude and contains 1.156 25 liters, which is about 1.222 U.S. liquid quart or 1.017 British imperial quart.
cubic centimeter (cm³ or cc): the CGS unit of volume, equal to 10^-6 cubic meter, 1 milliliter, or about 0.061 023 7 cubic inch.
cubic foot (ft³, cu ft, or cf): a traditional unit of volume in English speaking countries. One cubic foot equals 1728 in³, 1/27 yd³, 0.028 316 85 m³, or 28.316 85 liters. The cubic foot also holds about 7.4805 U.S. gallons or about 6.2288 British Imperial gallons.
cubic inch (in³or cu in): a traditional unit of volume in English speaking countries. One cubic inch equals 1/1728 = 5.787 037 x 10^-4 ft³, 16.3871 cm³, 16.3871 milliliters, 0.5541 U.S. fluid ounce, or 0.5767 British Imperial fluid ounce.
cubic meter (m³): the SI unit of volume, equal to 10⁶ cm³, 1000 liters, 35.3147 ft³, or 1.307 95 yd³. A cubic meter holds about 264.17 U.S. gallons or 219.99 British Imperial gallons.
cubic yard (yd³): a traditional unit of volume in English speaking countries. One cubic yard equals 27 ft³, 46 656 in³, 0.764 555 m³, or 764.555 liters. A cubic yard holds about 201.97 U.S. gallons or about 168.20 British Imperial gallons.
cubit: a historic unit of distance frequently mentioned in the Bible. The word comes from the Latin cubitum, "elbow," because the unit represents the length of a man's forearm from his elbow to the tip of his outstretched middle finger. This distance tends to be about 18 inches or roughly 45 centimeters. In ancient times, the cubit was usually defined to equal 24 digits or 6 palms. The Egyptian royal or "long" cubit, however, was equal to 28 digits or 7 palms. In the English system, the digit is conventionally identified as 3/4 inch; this makes the ordinary cubit exactly 18 inches (45.72 centimeters). The Roman cubit was shorter, about 44.4 centimeters (17.5 inches). The ordinary Egyptian cubit was just under 45 centimeters, and most authorities estimate the royal cubit at about 52.35 centimeters (20.61 inches).
cuerda (cda) [1]: a traditional unit of land area in Puerto Rico. The cuerda is equal to about 3930 square meters, 4700 square yards, 0.393 hectare, or 0.971 acre. Because the cuerda and the acre are so close to being equal, they are often treated informally as being equal.
cuerda (cda) [2]: a traditional unit of distance in Guatemala, equal to 25 varas or about 21 meters (roughly 69 feet). Since cuerda means a cord or rope in Spanish, this unit probably arose as the length of a measuring rope. The cuerda is also used as an area measure equal to 1 square cuerda or 625 square varas; this is about 440 square meters or 527 square yards.
cuerda (cda) [3]: a traditional unit of volume for firewood in Cuba, analogous to the U.S. cord (see above). A cuerda of firewood is equal to 128 cubic pies, 2.87 cubic meters, or 0.79 cord.
cumec: an informal unit of flow equal to 1 cubic meter per second or about 35.3147 cubic feet per second. The name was coined after the model of the cusec (see below).
cunit: a measure of wood volume used in forestry. One cunit (pronounced cue-nit) is a volume of timber containing 100 cubic feet (2.8317 cubic meters) of actual wood (excluding bark and air between the logs). The unit is used mostly for wood intended as pulpwood or firewood.
cup (c): a traditional unit of volume used in recipes in the United States. One cup equals 1/2 pint, or 8 fluid ounces. Technically, one cup equals exactly 14.4375 cubic inches or approximately 236.6 milliliters, not that anyone measures quite so precisely in the kitchen. American cooks use the same size cup for measuring both liquid and dry substances. In Britain, cooks sometimes use a similar unit called the breakfast cup.
curie (Ci): a unit of radioactivity. One curie was originally defined as the radioactivity of one gram of pure radium. In 1953 scientists agreed that the curie would represent exactly 3.7 x 10¹⁰ atomic disintegrations per second, or 37 gigabecquerels (GBq), this being the best estimate of the activity of a gram of radium. See also becquerel. The unit is named for Pierre and Marie Curie, the discoverers of radium.
cusec: a traditional unit of flow equal to 1 cubic foot per second or about 28.317 liters per second.
cut [1]: a traditional unit of length for yarn in Scotland and northern England. One cut equals 1/12 hank, a unit varying with the material. A cut of cotton yarn is 70 yards; a cut of wool is 46 2/3 yards.
cut [2]: a unit of concentration for shellac; see pound cut.
Cwt: traditional symbol for hundredweight.
cycle (c): an informal name for "cycle per second." The frequencies of radio signals and of alternating electric current were previously stated in cycles; thus the current in American homes is often described as "60-cycle" and a radio station might describe its signal frequency as "1040 kilocycles", really meaning 1040 kilocycles per second.
cycles per second (cps): a traditional unit of frequency equal to one per second, or one hertz. Almost all measurements of frequency are now stated in hertz, the SI unit.