L

labor: a traditional unit of area in Latin American countries. The labor is equal to the area of a square 1000 varas on a side, or 0.04 legua. Using the Texas standard for the vara (33 1/3 inches), this is equivalent to 177.136 acres or 71.685 hectares. The word labor means work in Spanish, as it does in English. As a unit it represents the area that could be cultivated by a single farmer, somewhat like the old English hide.
lakh or lac: a traditional unit of quantity in India, equal to 10⁵ or 100 000. In India the lakh is used commonly instead of the million and commas are used to isolate the number of lakh; for example, the number 5 300 000 is called 53 lakh and written "53,00,000". See also crore.
lambda: a metric unit of volume equal to the cubic millimeter (mm³) or microliter (µL). The lambda has been used in chemistry in measuring very small samples. The symbol is the lower case Greek letter lambda.
lambert (Lb or La): a unit of illumination. One lambert describes the illumination of a surface that emits or reflects one lumen per square centimeter. The lambert is a large unit, and practical measurements tend to be in millilamberts (mLb). The lambert has been replaced for the most part by the SI unit of illumination, the lux; 1 lambert equals 10⁴ lux and 1 millilambert equals 10 lux. (The only difference between "luminance" and "illumination" is the source of the light: if it comes from the surface we speak of "luminance," and if it comes from outside we speak of "illumination." Formerly luminance was measured in lamberts and illumination in phots. In the SI, both concepts are to be measured in lux.) The unit honors the German physicist Johann Lambert (1728-1777), who showed that the illumination of a surface is inversely proportional to the square of the distance from the light source.
land mile: the ordinary statute mile, equal to 5280 feet or 1609.344 meters, sometimes called a "land mile" to distinguish it from the nautical mile.
lane meter: a unit of deck area for "roll on/roll off" ships: cargo vessels designed so that containers or other cargo can be rolled on and off the decks of the ship. A lane is a strip of deck 2 meters wide. A lane meter is an area of deck one lane wide and one meter long, that is, 2 square meters (21.528 square feet).
langley (Ly): a CGS unit of heat transmission equal to one thermochemical calorie per square centimeter, or exactly 41.84 kilojoules per square meter (kJ/m²). Named for the American astronomer Samuel P. Langley (1834-1906), the langley is used to express the rate of solar radiation received by the earth.
lap: an informal unit of distance used in athletic competitions. In athletics ("track"), a lap is the length of one trip around a running track. This may vary from track to track, but at the level of serious competition most tracks have a standard length. In English speaking countries this was formerly 1/4 mile (1320 feet or 402.336 meters). Tracks used in most competitions today have a length of exactly 400 meters (1312.34 feet). In swimming, a lap is one tour (out and back) of the pool, a distance of exactly 100 meters (328.08 feet).
last: an ancient northern European unit used in measuring large quantities, either by mass or volume or both. "Last" is the German word for "load," a meaning which also survives in the English word "ballast." Generally the last is approximately 4000 pounds (about 1800 kilograms) as a mass unit or 80 bushels (about 3 cubic meters) as a volume unit. In the U.S., a last of wool was formerly 12 sacks at 364 pounds per sack, or 4368 pounds (1981.3 kilograms). In the Netherlands, the last is a metric unit of volume equal to exactly 3 cubic meters. In Germany, where the last was previously a volume unit equal to 2.819 cubic meters, it's also a metric unit of mass equal to 2 metric tons (about 4409 pounds). In Britain, a Riga last of timber is 80 cubic feet (2.265 cubic meters) of square-sawn timber.
lb, lbf, lbm: lb is the traditional English symbol for the pound, derived from the Latin libra, scale. The symbols lbf and lbm are used to distinguish between pounds of force and pounds of mass, respectively.
lea: a traditional unit of length for yarn, varying with the weight of the fibers in the yarn. Typically a lea of wool is 80 yards (73 meters); a lea of cotton or silk 120 yards (110 meters); and a lea of linen 300 yards (274 meters). In all these cases, a lea is equal to 1/7 hank. A lea is sometimes called a skein.
league: a traditional unit of distance. Derived from an ancient Celtic unit and adopted by the Romans as the leuga, the league became a common unit of measurement throughout western Europe. It was intended to represent, roughly, the distance a person could walk in an hour. The Celtic unit seems to have been rather short (about 1.5 Roman miles, which is roughly 1.4 statute miles or 2275 meters), but the unit grew longer over time. In many cases it was equal to 3 miles, using whatever version of the mile was current. At sea, the league was most often equal to 3 nautical miles, which is 1/20 degree [2], 3.45 statute miles, or about 5556 meters. The British definition of the league is 3 statute miles (about 4828 meters) on land or 3 nautical miles at sea. In the U.S., leagues have never been used on land and the only definition is 3 nautical miles. However, many occurrances of the "league" in English-language works are actually references to the Spanish league (the legua), the Portuguese league (legoa) or the French league (lieue). For these units, see below on this page.
leap: a traditional Welsh unit of distance equal to 6 feet 9 inches or 2.0574 meters.
legoa: the Portuguese league, equal to 3 milhas (Portuguese miles). This is equal to about 3.836 statute miles or 6174.1 meters.
legua [1]: the Spanish league. The traditional legua is equal to 5000 varas, which is close to 2.6 miles or 4.2 kilometers. Using the Texas definition of the vara, the legua is 2.6305 miles, 13889 feet, or 4233.4 meters. Using the traditional Spanish definition, it would be 2.597 miles, 13712 feet, or 4179.4 meters. Technically, this unit was abolished by Philip II in 1568, but it remained in rather wide use, especially in the Americas. During the late 18th and early 19th centuries, a league of 8000 varas (4.15 miles or 6680 meters) was legal in Spain. At sea, Spanish sailors used the usual marine league (3 nautical miles or 5556 meters) or Phillip's "geographical" league of 1/17.5 degree (3.429 nautical miles or 6350.5 meters).
legua [2]: a traditional Spanish unit of area equal to one square legua [1]. In Spanish-speaking Latin America and the southwestern states of the U.S. land was customarily measured in leguas, with 1 legua equal to 25 labors (see above) or 25 million square varas. Using the Texas definition of the vara as the starting point, the legua is 4428.4 acres, 6.919 square miles, 1792 hectares, or 17.92 square kilometers. A slightly larger figure, 4439 acres (1796 hectares), is used in California. Larger sizes, between 1800 and 1900 hectares, were formerly used in some parts of South America. In Mexico and Texas, this unit is often called a sitio.
length (lg): an informal unit of distance. The distance between competitors in horse races, boat races, and similar situations is naturally expressed in lengths, with one length equal to the average length of a horse, boat, etc. In horse racing, the length is actually interpreted as a unit of time equal to 1/5 second: the number of lengths by which one horse beats another is computed as 5 times the difference in their running times measured in seconds.
lethal dose (LD): a measure used in pharmacology to express the percentage of a population killed by a dose of the substance being studied. The measurement is often given as a subscript. For example, the potency of a drug or pesticide is commonly expressed by stating the size of the LD₅₀ dose: the amount of the substance which kills 50% of the test population.
li: a traditional unit of distance in China. Although current Western dictionaries usually describe the li as being approximately 1/3 mile or 500 meters, the late imperial governments of China used a li of 1800 ch'ih, which is 2115 feet, about 0.401 mile, or 644.65 meters. The well-known Confucian proverb actually says "a journey of a thousand li begins with a single step."
liang: a traditional Chinese weight unit. During the European colonial period the liang was equal to 1/16 catty, 1/12 pound, or about 37.8 grams; this made it the same as a tael. In modern China, the liang equals 1/10 jin or 10 qian; this is exactly 50 grams (1.7637 ounces).
libra or libbra (lb): a traditional unit of weight in Italian, Spanish, and Portuguese speaking countries. The libra was the Roman unit from which the English pound is descended; the symbol "lb" for the pound comes from this unit. The Roman libra contained only 12 unciae (ounces) and was about 0.722 English pound. The traditional Italian libbra was of similar size. The Spanish and Portuguese units are larger, generally in the range from 1.011 to 1.016 English pound (very close to 460 grams). The Spanish libra equals 16 onzas, and the Portuguese libra equals 16 onças. The word "libra" is sometimes used now for the kilogram, a much larger unit.
lieue: the French league. A variety of lieue units were used for land measurement in France, but generally these units were around 2.4-2.5 statute miles in length. In the 18th century, the legal unit was the lieue de poste, defined to equal 2000 toises or 2 milles (2.4221 miles or 3898 meters). In metric France the lieue is now considered to equal exactly 4 kilometers (2.4855 miles). See league (above). At sea, the lieue was often taken to equal 1/25 degree [2] or 2.4 nautical miles (4445 meters or 2.7619 miles); this unit was gradually replaced by the internationally recognized 3 nautical miles (5556 meters or 3.452 miles).
light watt: a unit measuring the relative power output of a light source. Calculating the power delivered in the form of visible light is rather complicated. For a monochromatic (single frequency) light source such as a laser, the power in light watts equals 683 V(l), where l is the wavelength of the light and V(l) is the relative power in watts per lumen (W/lm) required to produce a constant brightness sensation in the eye at wavelength l. Values of V(l) are defined by the International Commission on Illumination (CIE). The maximum value of V( l) is 1/683 W/lm = 1.464 mW/lm at the wavelength l = 555 nanometers (nm), the wavelength to which the eye is most sensitive. When the source delivers light over a range of frequencies (as a light bulb does), it is necessary to compute 683 times the integral of V(l) multiplied by the fraction of energy delivered at wavelength l. See also candela and talbot.
light year (ly): a unit of distance used in astronomy. One light year is the distance that light travels in one year through a vacuum (and, of course, most of the Universe is close to being a vacuum). Since light travels at 299 792 458 meters per second, and there are 31 556 925.9747 seconds in a year, one light year equals 9.460 528 405 x 10¹⁵ meters, or 9.460 528 405 trillion kilometers. One light year is approximately 5.880 trillion miles. See Length
ligne: a traditional unit of distance in French speaking countries, equal to 1/12 pouce (French inch) and corresponding closely to the English line [1]. The Swiss ligne is used throughout the world by watchmakers; it equals about 2.256 millimeters (0.0888 inch) and is divided into 12 douzièmes. In English the ligne is usually pronounced "lean" by those who don't hold to the French pronunciation.
ligula: a Roman unit of liquid volume equal to 1/48 sextarius or about 11.07 milliliters. The word literally means "a lick."
line (li) [1]: a traditional unit of distance, equal to 1/12 inch (about 2.1167 millimeters). For measuring the thickness of buttons, there is also a smaller line equal to 1/40 inch (0.635 millimeter). The line is called the ligne (see above) in French, the linea in Spanish, the linie in German, and the liniya in Russian.
line (li) [2]: a former name for the maxwell, the CGS unit of magnetic flux. The unit was called the line because magnetic fields were traditionally represented by lines depicting the direction of the field; the idea was to quantify the strength of these lines. This is a small unit, so fields were often measured in megalines; one megaline is equal to 0.01 weber.
linear foot (or "lineal" foot) (ft or lf): terms used loosely to describe a one-foot length of any long, narrow object. The correct term is linear foot; the word "lineal" refers to a line of ancestry, not to length. Boards, pipes, and fencing are typical objects measured and sold by the linear foot. In the moving industry, a linear foot is a one-foot length of a moving van, usually a volume of about 72 cubic feet (roughly 2 cubic meters). Occasionally the term "linear foot" is used as an alternate name for the board foot, but this is appropriate only if the board is 12 inches wide. See Length
link: a traditional unit of distance used by surveyors, equal to 0.01 chain. In Britain, one link is exactly 0.66 feet, or 7.92 inches, or approximately 20.12 centimeters. In the U.S., where 100- foot chains are more common, the link is the same as the foot.
liter or litre (L or l): the common metric unit of volume. The liter was originally defined to be the volume occupied by a kilogram of water, and the gram as the mass of a cubic centimeter of water. This would make the liter equal to exactly one cubic decimeter, that is, to the volume of a cube 0.1 meter (or 10 centimeters) on a side. Unfortunately, the physical objects constructed to represent the meter and kilogram disagreed slightly. As measured by the standard meter and standard kilogram, the standard liter turned out to be about 1.000 028 cubic decimeters. This discrepancy plagued the metric system for a long time. In 1901 an international congress accepted the discrepancy and formally defined the liter to be exactly 1.000 028 dm³. No one was particularly happy with such an awkward definition, so in 1964 the CGPM repealed the definition. In the SI, volumes are to be measured in cubic meters or power-of-ten multiples thereof, not in liters. However, the SI states that the liter "may be employed as a special name for the cubic decimeter." Throughout this dictionary, the liter is used as a name for exactly 1 cubic decimeter, 1000 cubic centimeters, or 0.001 cubic meter. In its new guise as the cubic decimeter, the liter is approximately 61.023 744 cubic inches. Compared to the customary volume units, the liter is a little more than a U. S. liquid quart (1.056 688 qt) but a little less than a U. S. dry quart (0.908 08 qt) or a British imperial quart (0.879 89 qt). Its name comes from a French volume unit, the litron, which was in turn derived from the Latin litra. Both the lower case letter l and the upper case L are accepted as symbols for the liter, but the U.S. Department of Commerce specifies that L be used, at least by businesses, to avoid confusion with the numeral 1. The unit is spelled liter in the U.S. and litre in Britain; there are many other spellings in various languages (see Spelling of Metric Units). See Volume
liter atmosphere (L atm): a unit of work or energy used in the study of confined gases. The behavior of gases is described, to a first approximation, by the ideal gas law PV = nRT. The ideal gas law is really an energy equation in which the left hand side, pressure P (in atmospheres) times volume V (in liters), measures the potential energy in the confined gas. One liter atmosphere is equal to 101.325 joules, 0.09605 Btu or 74.73 foot pounds.
livre: a traditional unit of weight in French speaking countries and in Greece. The livre corresponds to the English pound and to the Spanish libra (see above). The livre is divided into 2 marcs or into 16 onces. The French livre varied from market to market, but the official standard from about 1350 to the introduction of the metric system was the livre poids de marc or livre de Paris of 489.5 grams (1.079 English pounds). In modern France, the livre is used as a metric unit equal to exactly 500 grams or 0.5 kilogram (1.1023 pounds). The traditional Greek livre is also about 500 grams.
load: a traditional, generally informal, unit of volume. In landscaping and some construction trades a load often means a cubic yard (0.764555 m³). In ordinary language in the U.S., a load often means the volume of a pickup truck, a varying unit. In Britain prior to modern times, a load was sometimes a standardized unit, but it varied with the commodity being carried. A typical size was 40 bushels (roughly 1.4 cubic meters).
long hundredweight: the British hundredweight, equal to 112 pounds.
long ton: the traditional British ton, equal to 2240 pounds.
lot: a traditional unit of weight in German speaking countries, equal to about 1/2 ounce or 15 grams. "Lot" is the German word for a lead plumbbob, so the unit represents a small lead weight. The plural is lote.
lpi: abbreviation for lines per inch, a unit used to state the resolution of display devices (such as television or computer monitor screens), or to state the line spacing of printed pages.
lug [1]: an old English name for a rod (5.5 yards or 5.0292 meters). In some parts of England this unit represented a longer rod of 7 yards (6.4008 meters), a unit also called the great lug.
lug [2]: a shallow box or crate for produce such as cherries, grapes, or peaches. The size of a lug varies with the item it contains. Typical lugs hold about 16-28 pounds (7-13 kilograms) of produce in a volume of roughly 1/3 bushel (about 12 liters). This unit seems to be particularly common in produce markets in the midwestern U.S.
lumberg: an older name for the talbot, the unit of luminous (light) energy equal to 1 lumen second.
lumen (lm): the SI unit for measuring the flux of light being produced by a light source or received by a surface. The intensity of a light source is measured in candelas. One lumen represents the total flux of light emitted, equal to the intensity in candelas multiplied by the solid angle in steradians (1/(4·pi) of a sphere) into which the light is emitted. Thus the total flux of a one-candela light, if the light is emitted uniformly in all directions, is 4·pi lumens.
lumen hour (lm h): a unit of quantity of light, equal to one lumen of light flux continued for one hour. The lumen second (lm s) is defined similarly.
lunar month, lunation: names for the interval between two successive new moons, a unit of time equal to 29.53059 days. See month, Saros .
Lunar Distance: 1 LD = 384,401 km, the distance between Earth and the Moon. 1 LD also equals 0.00256 AU. MAG is the visual magnitude of the asteroid on the date of closest approach.
lusec: a unit of power used to express the performance or leakage of vacuum pumps. One lusec represents a flow of one liter per second at a pressure of one micrometer (or micron) of mercury, or 1 L·µmHg/s. Since "u" is sometimes used as a symbol for the micron, the name of the unit is an acronym for liter-micron/second. One lusec is equivalent to 1.3156 atm·cm³/s or 133.3 Pa·cm³/s, which is the same as 133.3 microwatts.
luster, lustre, lustrum: a traditional unit of time equal to 5 years. In ancient Rome the Lustrum was ceremony of expiation and purification for the whole population of the city, carried out every 5 years after the completion of the census. The use of luster or lustrum as a unit of time in English was fairly common in well- educated circles as long as "well-educated" meant classically educated; the unit has pretty much disappeared today.
lux (lx): the SI unit for measuring the illumination of a surface. One lux is defined as an illumination of one lumen per square meter. The difference between these light units may seem a bit subtle. Here's a review: the intensity of a light source is measured in candelas; the total light flux transmitted is measured in lumens (1 lumen = 1 candela·steradian); and the amount of light received per unit area is measured in lux (1 lux = 1 lumen/square meter). One lux is equal to approximately 0.09290 foot candle.