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calendar year
a civil unit of time, equal to 365 days or (in leap years) 366 days. In archaeology, climatology, and other sciences studying the earth over the last 40 000 years or so, a careful distinction must be made between calendar years and radiocarbon years (14C year).
c
a symbol for the velocity  of light. 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.
caliber
a unit used to express the bore of a gun, the bore is the inside diameter of the gun barrel. The diameter was stated in inches, so ".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.  The unit is often spelled "calibre" elsewhere. 
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  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 unit of heat energy, 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.
Calorie (kcal or Cal)
This unit is properly called the kilocalorie; it is also called the kilogram calorie or large calorie. It is often  distinguished from the small calorie by capitalizing its name and symbol. The large calorie, or rather kilocalorie, 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.
candela
the SI base unit for measuring the intensity of light. Candela is the Latin word for "candle." 
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" now 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.
carat
a unit of mass used for diamonds and other precious stones. Originally spelled karat, the word comes from the Greek keration, a carob bean; carob beans were used as standards of weight and length in ancient Greece. 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.
carton
a small container. The size of a carton is usually not standardized, but certain sizes are customary. In the U.S. citrus fruit industry, a standard carton is equal to 1/2 box or 0.8 bushel (28.191 liters).
category
the ranking of a hurricane on the Saffir-Simpson scale, used by the U.S. National Weather Service. 
Celsius
degree Celsius, the word "degree" is often omitted in informal statements of temperature.
Celsius heat unit
a unit of heat energy equal to the energy required to raise the temperature of one pound of water by 1C at standard atmospheric pressure. approximately 453.59 IT calories (see above), or 1.8991 kilojoules. The unit is also called the centigrade heat unit.
cent
a unit used in music when it is necessary to specify the ratio in frequency between two tones with great precision.
centi- (c-)
a metric prefix meaning one hundredth, or 0.01. The prefix comes from the Latin word centum for one hundred.
centigram
a metric unit of mass equal to 10 milligrams or about 0.154 grain.
centiliter (cl or cL)
a common metric unit of volume. One centiliter equals 10 cubic centimeters; this is about 0.610 24 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.
centimeter (cm)
the basic unit of distance in the former CGS version of the metric system, equal to 0.01 meter. One centimeter is about 0.393 700 787 inch.
centimeter of mercury (cmHg)
a traditional unit of pressure equal to 10 mmHg, 1.333 22 kilopascal, or about 0.193 pounds per square inch.
centimillion
a word sometimes used incorrectly to mean 100 million (108). This is a serious misuse of the metric prefix centi-, which means 1/100, not 100. The number 100 million could be called a hectomillion or (at least in the U.S.) a decibillion.
centinewton
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 1.019 72 gf.
centisecond
a unit of time equal to 0.01 second or 10 milliseconds. Centiseconds are frequently used in the study of human speech to measure precisely the length of sounds.
centner
The English name for a German weight or mass unit,  equal to 50 kilograms or about 110.231 pounds.
 
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 was twice the size of the centner used in western Europe.
century
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 centurion.
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 included the years 1901-2000. In the other direction, the Fifth Century BC included the years 500-401 BC.
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. Cetane is the name of a hydrocarbon compound whose molecules contain 16 carbon atoms and 34 hydrogen atoms, the 16 carbons being arranged in a long chain.
chain
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 1/80 mile, 1/10 furlong, 22 yards, or 66 feet (20.1168 meters). The traditional length of a cricket pitch is 1 chain. 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 . Based on Texas version, the vara chain of 2 varas (55.556 ft) was used in surveying Spanish land grants. In the metric world, surveyors often use a chain of 20 meters (65.617 ft).
chain number
a size designation for roller chains, such as the drive chains of bicycles or motorcycles. These chains are traditionally designated by a three-digit number. The first digit specifies the pitch, the distance between pins, in eighths of an inch; the second and third digits specify the width of the rollers in 80ths of an inch. Thus a 425 chain has a pitch of 4/8 inch and a roller width of 25/80 inch.
chalder or chaldron (chd)
an old British unit of volume or weight used for dry commodities such as 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).
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 newer coding system designed to include 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.
chi (or ch'ih)
a unit of distance used in China. The chi equals 10 cun (t'sun), 35.814 centimeters, or 14.1 inches. There are 1800 chi in a li.
chiliad
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.
another name for a millennium (1000 years).
chin
one of several spellings in English for the jin, a traditional Chinese weight unit.
chip
in New Zealand, another name for a punnet.
choi
a name used in Thailand and Laos for the viss. The word is sometimes spelled joi in English.
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).
a traditional Scottish unit of volume equal to 2 mutchkins or 1/2 Scots pint. The choppine is equivalent to about 52.1 cubic inches, 1.80 U.S. liquid pints, 1.50 British imperial pints, or 854 milliliters.
ci
a symbol for the cubic inch. This symbol was formerly used in the U.S. in stating the engine displacements of motor vehicles; those measurements are now made in liters.
cinque
an old English word for the number 5, pronounced "sink" and derived from the French number 5, cinq. 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 pi radians or 360 degrees.
circular inch, circular mil (cmil)
informal units of area. A circular inch [mil] is the area of a circle one inch [mil] in diameter. A circle of diameter d has an area of pid2/4, so the circular inch is equal to approximately 0.785 398 square inches or 5.067 07 square centimeters, and the circular mil is equal to approximately 0.785 398 square mils or 506.707 square micrometers.
civil year
a year as measured by the conventional (Gregorian) calendar, equal to 365 days in most years but 366 days in a leap year.
click
U.S. military slang for the kilometer (about 0.621 mile). Also spelled klick or klik. This unit became popular during the Vietnam War, but it was 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).
clothyard, clothier's yard
an alternate name for the ell. The English ell is 45 inches (1.143 meters), but the "clothyard arrows" used with longbows in late medieval times were closer in length to the 37-inch Scottish ell.
clove
an old English unit of weight. A clove is usually considered equal to 1/2 stone or 1/16 hundredweight (7 pounds or 3.175 kilograms), but in the past, the clove varied from 6.25 to 8 pounds.
CmA
a unit of relative electric current used especially in connection with nickel metal hydride (NiMH) storage batteries. The symbol designates the current flow per hour, into or out of the battery, as a fraction of the battery's rated capacity.
coffee measure
a flat-bottomed scoop or spoon used to measure coffee in U.S. 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 rendering index (CRI)
a scale used in engineering to measure the ability of an artificial lighting system to show the "true" colors of objects, that is, the colors those object display in natural daylight out of doors. The scale is from 0 to 100, with higher numbers representing a higher degree of fidelity of color.
color temperature (CCT)
a measure of the overall "color" of a light source. The measurement is obtained by comparing the spectrum, or mix of colors (wavelengths of length), produced by the light source to the spectrum of a "black body," a theoretical object that absorbs all radiation falling on it.  Lower temperatures indicate more red and yellow light, higher temperatures more blue.
color units
several systems have been devised to measure colors. For most of us not directly concerned with dyes, paints, or inks, the subject was academic until recently, but now computers require precise methods for describing the colors to be displayed or printed. These methods typically use three variables, reflecting the fact that the human eye has three types of color sensor. 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. Usually 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 Carolina blue background of this page are R=153, G=204, and B=255. 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. In the opposite direction, the color becomes less bright, becoming black at value 0.
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."
commercial acre
a unit of area used in U.S. real estate, equal to exactly 36 000 square feet or about 0.826 45 ordinary acre (0.334 45 hectare). It is legal to sell land by the commercial acre in many U.S. states, although most consumers are not aware of the smaller size of the unit.
cone, cone number
a measure of temperature used by potters. Pyrometric cones are cone-shaped objects designed to soften and bend after absorbing a specific amount of heat. Potters place these cones in the kiln and observe them through peepholes; when the cone bends all the way over the proper amount of heat has been delivered to the pottery being fired. Although cones bend within narrow temperature ranges there is not a simple relationship between cone number and temperature.
cong
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. In the nineteenth century, the congius was used in British medicine and pharmacology as a name for the British Imperial gallon (4.546 09 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. A heat pump that 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 divided by 3.412.
cord
a traditional unit of volume used to measure stacked firewood. 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. One cord is a volume of 128 cubic feet, about 3.6247 cubic meters, or 3.6247 steres.
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 charged particles, such as electrons, protons, or ions. 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
the quantity stated represents a reliable count. a traditional unit measuring the texture of a fabric, equal to the number of eads per inch. A 100 count fabric has 39.37 threads per centimeter.
cousins
like most languages, English 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. This means n-th cousins have n + 1 generations in each of their descents from the common ancestor.
cover
a traditional Welsh unit of area, standardized in the British system to be exactly 2/3 acre (about 0.2698 hectare). The word is an Anglicized version of the Welsh name cyfair for the unit.
cpi
a common abbreviation for characters per inch, used in printing. The unit is also called pitch.
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 (0.01C) and pressure (1 atmosphere); this is about 89.885 milligrams. The name comes from an ancient Greek word for a barleycorn.
crore
an 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.
cuadra
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 is equal to 150 varas (roughly 130 meters or 410 feet).
cuartillo
a traditional Spanish unit of volume comparable to the liter or the English quart. 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. liquid gallons or about 6.2288 British Imperial gallons.
cubic inch (in3, cu in, or ci)
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. liquid 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. liquid gallons or about 168.20 British Imperial gallons.
cubit
a historic unit of distance in the Bible. The word comes from the Latin cubitum, "elbow,"  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).
cun
chinese lenth unit, 1/10 chi, see t'sun.
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. The unit is used mostly for wood intended as pulpwood or firewood.
cup
a traditional unit of volume used in recipes in the United States. One cup equals 1/2 (liquid) 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 Canada, a cup is equal to 8 Imperial fluid ounces (13.8710 cubic inches or 227.3 milliliters). In Britain, the breakfast cup, equal to 10 Imperial fluid ounces. 250 milliliters, commonly used in recipes in Australia.
curie
a unit of radioactivity. One curie was originally defined as the radioactivity of one gram of pure radium.
cusec
a traditional unit of flow equal to 1 cubic foot per second or about 28.317 liters per second.
cut
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.
CV or cv
a common symbol for the metric horsepower, standing for the French name cheval vapeur or the Spanish name caballo de vapor.
Cwt or cwt
traditional symbol for the hundredweight.
cycle
The frequencies of radio signals and of alternating electric current were previously stated in cycles; a radio station might describe its signal frequency as "1010 kilocycles", really meaning 1010 kilocycles per second. a traditional unit of frequency equal to one cycle per second, or one hertz. Almost all measurements of frequency are now stated in hertz, the SI unit.

 

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