Q

Q: a metric unit of distance equal to exactly 0.25 millimeter (9.8425 mils) used by typographers and page designers in Japan, in Germany and in other countries in preference to the traditional point [2]. Some graphics software is referring to this unit as the kyu.
QD: abbreviation for the Latin quaque die, once a day, a unit of frequency traditionally used in medical prescriptions.
QH: abbreviation for the Latin quaque hora, once an hour, a unit of frequency traditionally used in medical prescriptions.
qian: a traditional Chinese weight unit. In modern China the qian is equal to 0.1 liang, or exactly 5 grams (0.1764 ounce).
QID: abbreviation for the Latin quater in die, four times a day, a unit of frequency traditionally used in medical prescriptions.
quad: a unit of energy equal to 10¹⁵ (one U.S. quadrillion) Btu or about 1.055 exajoules (EJ) or 293.07 terawatt hours (TWh).
quadbit: a unit of information equal to 4 bits or 1/2 byte. This unit is used in telecommunications, where data is frequently transmitted in quadbits. In other contexts, the same unit is called a tetrad, a nibble, or a hexit.
quadrant (quad) [1]: a unit of angle measure, equal to 1/4 circle, pi/2 radians, 90°, or 100 grads.
quadrant (quad) [2]: a unit of distance equal to the distance from the North Pole to the Equator. The metric system was originally designed to make this distance exactly 10 million meters (6213.71 miles). The actual meter comes close to the design. Using the conventional figures of 12,756 kilometers for the equatorial diameter of the earth and 12,714 kilometers for the polar diameter, and assuming the earth to have elliptic cross section, the length of the quadrant is about 10 001 300 meters (6214.52 miles). In principle, the quadrant is divided into 5400 nautical miles; in fact, 5,400 international nautical miles equal 10,000,800 meters.
quadrat- (q-): a German prefix meaning "square." For example, the square kilometer is the quadratkilometer (qkm or km²) in German.
quadrennium: a traditional unit of time equal to four years.
quadrumvirate: a unit of quantity equal to 4. The word was coined on the pattern of triumvirate.
quadruplet: a group of 4 items, especially 4 identical items; the word is also used for one member of the group.
quantum: a unit of energy used in physics. At the small scales studied in particle physics, energy occurs only is discrete packets or units called quanta. The amount of energy in a quantum depends on the frequency of the radiation carrying the energy; it is equal to the frequency (in hertz) multiplied by Planck's constant, 6.626069×10^-34joule second (J s).
quart (qt) [1]: a traditional unit of volume, so-called because it equals exactly 1/4 (one quarter) of a gallon. However, there are several possible gallons to consider: [i] in the U. S. customary measure system for liquid volumes (such as milk, for example), one quart is exactly 57.75 cubic inches, 32 fluid ounces, or approximately 0.9463529 liters; [ii] in the U. S. customary measure system for dry volumes (pecans or strawberries, for example), one quart is 67.201 cubic inches, or approximately 1.101 221 liters; [iii] finally, in the British imperial system, used for both liquid and dry commodities, one quart is 69.354 cubic inches, 40 fluid ounces, or exactly 1.136 5225 liters. (Units in the British imperial volume system are about 20% larger than U. S. liquid units, and about 3.2% larger than U. S. dry units.) In all cases, the quart equals 2 pints. See Volume.
quart (qt) [2]: a unit of volume, smaller than the standard quart [1], used for measuring wine. Wine bottles have often been called "quarts," although they were smaller than standard quarts. In the U.S., wine was often measured by the champagne quart, which contains only 26 U.S. fluid ounces instead of 32. This is equivalent to about 46.92 cubic inches or approximately 768.912 milliliters. In Britain, wine was sold by the reputed quart. Following the establishment of Imperial measure, the reputed quart was fixed at 2/3 imperial quart, which is equivalent to exactly 26 2/3 fluid ounces, about 46.24 cubic inches, or 757.682 milliliters. However, these measures now disappeared in favor of the international wine bottle containing exactly 750 milliliters.
quarter (qtr or Q or Qr) [1]: a traditional unit of weight equal to 1/4 hundredweight. In Britain, one quarter equals 28 pounds (12.7006 kilograms); in the United States, one quarter equals 25 pounds (11.3398 kilograms). In the U.S., "quarter" is also used informally to mean 1/4 ton, or 500 pounds (226.80 kilograms).
quarter (qtr or Q) [2]: a civil unit of time, equal to 3 months or 1/4 year. The quarter is widely used as a time unit in business and economics. Given the layout of the Julian/Gregorian calendar in civil use throughout the world (see year [2]), the quarter varies in length from 90 to 92 days depending on its starting date.
quarter (qtr or Q) [3]: a unit of angle measure equal to 1/4 circle; another name for a quadrant.
quarter (qtr or Q) [4]: a unit of angle measure, sometimes used at sea, equal to 1/4 of a compass point (see point [1]). In this use, a quarter equals 2°48'45" = 2.8125° or pi/64 = 0.049087 radian.
quarter (qtr or Q) [5]: a traditional unit of volume used for measuring grain. A quarter of grain is 8 bushels (about 281 liters, based on the U.S. bushel, or 291 liters, based on the British imperial bushel), perhaps because this quantity of grain weighs roughly 1/4 ton.
quarter (qtr or Q) [6]: an informal unit of distance equal to 1/4 mile, 2 furlongs, or 402.336 meters. This unit is used in athletics and horse racing.
quarter (qtr or Q) [7]: an informal unit of time equal to 1/4 hour or 15 minutes. This unit occurs in informal expressions of time, such as "quarter after 10" for 10:15.
quarter (qtr or Q) [8]: a unit of relative time used in sports, equal to 1/4 the total playing time of a competition. A quarter is 15 minutes in American football, 12 minutes in professional basketball.
quarter (Q) [9]: a unit of distance equal to 0.25 millimeter. See Q, above, and point [2].
quartern [1]: an old English word for a quarter of anything, "quartern" has been used to represent 1/4 of various units. In the U.S., the term seems to have been used mostly as an alternate name for the gill, which is equal to 1/4 pint or about 118.3 milliliters.
quartern [2]: a traditional English unit of weight equal to 1/4 stone, 3.5 pounds, or about 1.5876 kilogram. See also quartern-loaf, below.
quartern [3]: a traditional unit of volume for dry goods equal to 1/4 peck or 2 quarts. This corresponds to 2.2731 liters in the British imperial system or 2.2025 liters in the U.S. system. The unit is much more common in Britain.
quartern [4]: a traditional English unit of quantity equal to 25, or 1/4 of 100.
quartern-loaf: a traditional English unit of weight for bread. A quartern-loaf is made from a quartern [2] of flour. The finished loaf usually weighs about 4 pounds (1.81 kilograms); as a result a quartern is sometimes described as a weight of 4 pounds, somewhat in conflict with the definition of 1/4 stone given above.
quarter tone: a unit used in music to describe the ratio in frequency between notes. The quarter tone is 1/24 octave, the basic interval in a 24-tone scale. Two notes differ by a quarter tone if the ratio in their frequencies is 2^1/24 = 1.0293.
quartet: a unit of quantity equal to 4.
quartile: a statistical unit equal to 25 percentiles, or 1/4 of a ranked sample. See percentile.
quarto [1]: a traditional Italian unit of volume, equal to about 73.6 liters or 2.60 cubic feet.
quarto [2]: a traditional Portuguese unit of volume, not related to the Italian quarto nor to the English quart. The Portuguese quarto equals 2 oitavos, which is about 3.46 liters or 0.92 U.S. gallon. There are 16 quartos in a fanega, 124 in a pipa.
quarto [3]: in English, quarto is a page size; see -mo.
quaver: a unit of relative time in music equal to 1/8 whole note or 1/16 breve.
Quevenne scale: see degree Quevenne.
quinquennium: a traditional unit of time equal to five years.
quintal (q) [1]: a traditional unit of weight in France, Portugal, and Spain. Quintal is also the generic name for a historic unit used in commerce throughout Europe and the Arab world for more than 2000 years. The unit began as the Latin centenarius, meaning "comprised of 100" because it was equal to 100 Roman pounds (libra pondo). The centenarius passed into Arabic as the cantar or qintar and then returned to Europe through Arab traders in the form quintal. The German zentner and English hundredweight are familiar forms of this same unit in northern Europe. The traditional French quintal equaled 100 livres (48.95 kilograms or 107.9 pounds), but today the word "quintal" in France usually means a larger metric unit (see next entry). The Spanish quintal is 100 libras (about 46 kilograms or 101 pounds). The Portuguese quintal is larger; it is equal to 128 libras (about 129.5 pounds or 58.75 kilograms). "Kwintal" is the English pronunciation given in standard English dictionaries, but "kintal" (closer to the Spanish pronunciation) and "kantal" (closer to the French) are also used.
quintal (q) [2]: a common metric unit of mass equal to 100 kilograms or approximately 220.4623 pounds. Notice that the metric ton is roughly equal to its non-metric predecessors, but the metric quintal is about twice the size of the traditional quintal.
quintet: a unit of quantity equal to 5.
quintile: a statistical unit equal to 20 percentiles, or 1/5 of a ranked sample. See percentile.
quintuplet: a group of 5 items, especially 5 identical items; the word is also used for one member of the group.
quire (qr): a traditional unit of quantity used for counting sheets of paper. The word is from Latin, meaning "by fours." A quire was originally comprised of 24 sheets cut from four of the large sheets produced by the paper maker. In modern use a quire is often reckoned as 25 sheets, so that a ream of 20 quires is now 500 sheets rather than the traditional 480.
Q unit: a unit of heat energy equal to one quintillion (10¹⁸) Btu, 1000 quads, or about 1.055 zettajoules (ZJ).