P

pace: a traditional unit of distance, considered equal to the length of a person's "full" pace, that is, the distance between two successive falls of the same foot. Thus one pace equals two steps. The Romans counted 1000 paces in a mile, with each pace being a little over 58 inches (or about 148 centimeters). In more recent times, the pace has usually been defined as exactly 5 feet (or 152.4 centimeters); this unit is called the great pace or geometrical pace. In the military, however, the term "pace" is often used as an alternate name for the step; see military pace.
pack (pk): a commercial unit specifying the number of items per package. In retail trade, packages containing 4 items (for example) are often described as "4 pack" or "4pk." The symbol pk is also used for the peck (see below).
packen: a traditional Russian unit of weight equal to 1200 funte, 30 pudi (see pud below), 1083 pounds, or 491.4 kilograms.
pair (pr): a unit of quantity equal to 2. The word is from the Latin paria, meaning (two) equals.
palm [1]: a traditional unit of distance equal to the width of a person's palm. In the ancient world, the palm was considered equal to 4 digits or 1/6 cubit, which would be about 3 inches or 7.5 centimeters. This unit was used very commonly in medieval and early modern Britain. Similar units, all equal to 1/4 the local "foot" unit, were used throughout northern Europe.
palm [2]: In the U.S., perhaps due to familiarity with the Spanish palmo (see next entry), the palm is more often an informal unit equal to 9 inches (22.86 centimeters); this makes the U.S. palm an alternate name for the span.
palm [3]: a name sometimes used in Dutch for the decimeter (10 centimeters, or about 3.937 inches).
palmo: a traditional unit of distance in Spain and Portugal. The traditional Spanish palmo equals 9 pulgadas (see below) or 1/4 vara: this is about 20.9 centimeters in Spain and a little more than that in Spanish Latin America. In Texas, 1/4 vara comes to 8 1/3 inches (21.17 centimeters). Under the metric system in Spain, the palmo is an informal unit equal to 20 centimeters. The Portuguese palmo equals 0.1 braça or about 22.0 centimeters (8.66 inches). These units are based on the length of a person's hand, from the wrist to the tip of the middle finger.
parasang: a historic unit of distance comparable to the European league. The unit originated in Persia but was used throughout the ancient Middle East and Mediterranean. It was equal to roughly 3.5-4.0 miles or about 6 kilometers.
Paris foot: an English name for the French royal foot (pied de roi, see below).
parsec (pc): a non-metric unit of distance used in astronomy. As the Earth makes its orbit around the Sun, nearby stars appear to shift their positions in relation to the background of distant stars. The shift, called the parallax of the star, is very small, less than one arcsecond, (.7 arcsec) even for the nearest star. If it can be measured, however, then simple trigonometry can be used to find the distance from the Earth to the star. One parsec is the distance at which a star appears to shift its position by one arcsecond over the course the time (about 3 months) in which the Earth moves a distance of one astronomical unit (au) in the direction perpendicular to the direction to the star. Using this unit makes it easy to compute distances: the distance to a star, in parsecs, is simply one divided by the amount of the parallax. If the parallax is 0.01 seconds, the distance is 100 parsecs. The length of a parsec, divided by one astronomical unit (the radius of the Earth's orbit) is the trigonometric function of the parallax called the cotangent; from this relation we can compute that one parsec equals 206 264.8 au. This is the same as 3.261 631 light years, 30.85678 petameters (30.85678 x 10¹² kilometers), or about 19,173,510,000,000 or 19.173 × ¹² miles,. See Length Converter
particle flux unit (pfu): a unit used to measure the rate at which energetic particles, such as protons, are received by spacecraft. These flux rates are a major component of the "space weather," the environment in which satellites and other spacecraft operate. One pfu is a rate of one particle per square centimeter of detector area per steradian of solid angle scanned per second of time. (A steradian is about 7.96% of a sphere.) In SI units, 1 pfu = 10^-4 m^-2sr^-1s^-1.
part per billion (ppb): a unit of proportion equal to 10^-9.
part per million (ppm): a unit of proportion equal to 10^-6.
part per quadrillion (ppq): a unit of proportion equal to 10^-15.
part per thousand (ppth or ppt): a unit of proportion equal to 0.001, also called per mill (see below).
part per trillion (ppt): a unit of proportion equal to 10^-12.
pascal (Pa): the SI unit of pressure. The pascal is the standard pressure unit in the MKS metric system, equal to one newton per square meter or one "kilogram per meter per second per second." Sounds impressive, but in traditional English terms a pascal is only 0.000 145 pounds per square inch (0.020 885 lbf/ft² or 0.007 50 mmHg). Thus pressure is more commonly measured in kilopascals (kPa), with 1 kPa = 0.145 lbf/in². Air pressure is measured in hectopascals (hPa), with 1 hPa = 1 millibar. The unit is named for Blaise Pascal (1623-1662), French philosopher and mathematician, who was the first person to use a barometer to measure differences in altitude. See Pressure.
pascal second (Pa·s): the SI unit of dynamic viscosity, equal to 10 poise or 1000 centipoise. Some scientists want to call this unit the poiseuille (Pl), but that name has not been accepted by the General Conference on Weights and Measures. See Dynamic Viscosity
pcf, pci: symbols for pound per cubic foot or per cubic inch, traditional engineering units of density. 1 pcf = 16.018 46 kg/m³ and 1 pci = 1728 pcf = 27 679.90 kg/m³.
pé: the traditional Portuguese foot, equal to 12 polegadas or about 33.324 centimeters (13.12 inches).
pebi- (Pi-): a binary prefix meaning 2⁵⁰ = 1 125 899 906 842 624. This prefix, adopted by the International Electrotechnical Commission in 1998, replaces peta- for binary applications in computer science. The prefix is a contraction of "petabinary."
peck (pk): a traditional unit of volume, formerly used for both liquids and solids but now used mostly for dry commodities such as grains, berries, and fruits. A peck is 2 gallons, 8 quarts, or 1/4 bushel. In the U. S. customary system, a peck holds 537.605 cubic inches, or approximately 8.8098 liters. In the British imperial system, a peck is a little larger, holding 554.84 cubic inches, or approximately 9.0923 liters. The word "peck", originally spelled "pek", comes from the name of a similar French unit. See Volume
-penny: an ending added to a number to indicate the size of a nail, as in "sixpenny nail" or "tenpenny nail." It's not clear exactly how this terminology began, although the usual guess is that tenpenny nails originally cost ten pence per hundred. There is, very roughly, a linear relation between the size designations and length: an n- penny nail is roughly (1/2) + (1/4)n inches long. This makes the tenpenny nail about 3 inches long, the eightpenny about 2.5 inches, and so on.
pennyweight (dwt or pwt): a unit of weight in the traditional troy system (see pound [2]), equal to 24 grains or 1/20 troy ounce. One pennyweight is approximately 1.5552 gram. The d in the traditional symbol dwt is from the Latin word denarius for the small coin which was the Roman equivalent of a penny. (The letter d is also the traditional symbol for the penny in the English monetary system.)
pentad [1]: a unit of quantity equal to 5.
pentad [2]: a unit of time equal to 5 days. This unit is common in meteorology, where forecasts are frequently made for periods of 5 days at a time.
per annum (PA): a traditional unit of frequency equal to once a year.
percent (% or pct): a unit of proportion, equal to 0.01. The word is Latin, meaning "by the hundred." The symbol % can be placed after any number; mathematically, its effect is an immediate division by 100.
percentile: a unit used in statistics to describe a portion of the individuals or events being studied. Suppose the data are arranged by numerical scores, from highest to lowest. A score belongs to the 78th percentile, for example, if it is greater than 78% of the scores but it is not greater than 79% of the scores. This procedure divides the scores into 100 percentiles, numbered 0th through 99th. A group of 10 consecutive percentiles is called a decile; 25 consecutive percentiles are called a quartile.
perch [1]: an alternate name for the rod (16.5 feet or 5.0292 meters), introduced in the twelfth century by the Norman conquerors of England. The word perch ( perche in French: see below) comes from the Latin pertica (pole). The Romans also had a distance unit called the pertica, but it was shorter: 10 Roman feet (9.71 English feet or 2.96 meters). See Length Converter.
perch [2]: a unit of area equal to one square perch [1]. A perch of area covers exactly 272.25 square feet or about 25.292 85 square meters.
perch [3]: a traditional unit of volume for stone and other masonry. A perch of masonry is the volume of a stone wall one perch (16.5 feet) long, 18 inches high, and 12 inches thick. This is equivalent to exactly 24.75 cubic feet, 0.916 667 cubic yard, or about 0.700 842 cubic meter.
perch [4]: a traditional unit of distance in Ireland standardized at 21 English feet (6.4008 meters) or 14/11 English perch or rod. Since the Irish perch was 27.27% longer than the English, the Irish chain, furlong, and mile were longer in the same proportion..
perche or perch [1]: a traditional unit of distance in French North America. The perche equals 18 pieds (see below) or 3 toises. By legal definition in Canada this equals 19.1835 English feet or 5.847 13 meters.
perche or perch [2]: a traditional unit of area in French North America, equal to one square perche [1]. A perche of area is therefore equal to 0.01 arpent, about 368.007 square feet (40.8896 square yards) or 155.330 square meters.
per diem (PD): a traditional unit of frequency equal to once a day.
perfect ream: a unit of quantity for paper. An ordinary ream is 480 or 500 sheets; a perfect ream is 516 sheets. The additional amount is to allow for sheets which may be spoiled in shipment.
perm: a traditional unit of water vapor permeability, that is, the ability of a material to permit the passage of water vapor. If we want to keep things dry, we wrap them in something having low permeability. A material has a permeability of one perm if it allows transmission of one grain of water vapor per square foot of area per inch of mercury (inHg) of pressure difference per hour. The value depends somewhat on temperature, however. At 0 °C, one perm equals about 5.721 x 10^-11 kilograms per square meter per pascal per second (kg/m²·Pa·s) or about 0.2060 mg/m²·Pa·h; at room temperature the equivalent is about 5.745 x 10^-11 kg/m²·Pa·s. The lower the perm value, the better the vapor barrier.
per mensem: a traditional unit of frequency equal to once a month.
per mill, per mil: a unit of proportion, equal to 0.001. Unlike percent, per mill is written as two words. Its symbol, not available to most web browsers, is like the percent symbol but with two zeroes in the denominator (roughly, ⁰/₀₀). The spelling "per mill" seems to be more common in the U.S., "per mil" being more common in Britain.
perm inch: a traditional unit of water vapor permeance. The perm value (see above) does not depend on the thickness of the material used as a water barrier. The permeance is the product of the perm value and the thickness, measured in inches. One perm inch equals about 1.453 x 10^-12 kg/m·Pa·s at 0 °C or about 1.459 x 10^-12 kg/m·Pa·s at room temperature.
person hour: a gender-nonspecific version of man hour, a unit of labor equal to the work of one person for one hour.
pes: the Roman foot, equal to 29.67 centimeters (about 11.68 inches). The pes was divided into 12 unciae (inches). There were 5 pes (or pedes) in 1 passus (pace, see above), 10 in a decempeda, 625 in a stadium, and 5000 in the Roman mile.
peta- (P-): a metric prefix denoting 10¹⁵ (one U.S. quadrillion). One parsec, for example, equals 30.857 petameters. The prefix is drawn from a Greek root which means "spreading out"; our word petal has the same root. The prefix is usually pronounced pet-a, with a short "e" sound, rather than pee-ta. See Prefix
petabecquerel (PBq): a unit of radioactivity equal to 10¹⁵ atomic disintegrations per second or 27.027 curies.
petajoule (PJ): a metric unit of energy. One petajoule equals 947.817 billion Btu, 277.7778 gigawatt hours, or about 9.48 megatherms.
petameter (Pm): a metric unit of distance equal to 10¹² kilometers. This is equivalent to about 621.371 billion miles or 0.1057 light year. The distance from the earth to the nearest star (other than the sun) is about 40 petameters.
pferdestärke (ps): German word for horsepower, meaning the metric horsepower. The symbol ps is used for horsepower in both the Japanese and German automotive industies.
pfu: symbol for the particle flux unit (see above).
pfund (pfd): a traditional German weight unit corresponding to the English pound (see below). The pfund is equal to 16 unze or 32 lot. Traditionally the pfund varied in size from market to market, and the various German states adopted different standards, ranging from something close to the English pound (454 grams) to the Viennese pfund at about 1.2 pounds (560 grams). When Germany was unified in the late nineteenth century, the pfund was redefined as a metric unit equal to exactly 500 grams (about 1.102 31 pound). There's no change in the plural.
pH: a logarithmic unit used to measure the acidity or alkalinity of a chemical solution. The properties of a liquid solution we call "acid" are caused by the presence of hydrogen ions (H⁺). The pH of a solution is a measure of the concentration of these hydrogen ions. Technically, the pH of a solution is defined to be the negative logarithm of the concentration, measured in moles per liter. This unit is inverted in the sense that lower pH readings correspond to greater acidity, and therefore more hydrogen ions. Lowering the pH by 1.0 means multiplying the ion concentration by a factor of 10. Mathematically the scale is open at both ends, but in practice pH values usually fall in the range from 0 to 14. A neutral solution (neither acidic nor alkaline, like pure water) measures 7.0 pH. Numbers below 7 indicate increasing acidity, while numbers above 7 indicate increasing alkalinity. The pH scale was invented by S. P. L. Sorenson in 1909.
phon: a unit of sound loudness closely related to the decibel. Decibels are used for objective measurements, that is, they measure the actual pressure of the sound waves as recorded using a microphone. Phons are used for subjective measurements, that is, measurements made using the ears of a human listener. A sound has loudness p phons if it seems to the listener to be equal in loudness to the sound of a pure tone of frequency 1 kilohertz and strength p decibels. A measurement in phons will be similar to a measurement in decibels, but not identical, since the perceived loudness of a sound depends on the distribution of frequencies in the sound as well as the pressure of the sound waves. In the U.S., sound loudness is frequently measured in sones rather than phons: a sound of loudness s sones has loudness 10 log₂s + 40 phons. In English the unit is pronounced "fon" rather than "phone."
phot (ph): the CGS unit of illumination, equal to one lumen per square centimeter or 10 000 lux. See also lambert. The name is usually pronounced to rhyme with "not" rather than "note."
photon [1]: a quantum of light energy, or, more generally, of any form of electromagnetic energy having a single wavelength, direction, and polarization.
photon [2]: a former name for the unit of retinal illuminance now called the troland.
pi: a very famous mathematical unit. The circumference of a circle is equal to pi multiplied by the diameter, so pi is equal to the ratio between the circumference and diameter of a circle, any circle. It turns out that pi is an irrational number, which means that its decimal expansion is nonterminating and nonrepeating. To 25 significant digits, pi equals 3.141592653589793238462643. See pi for a usefull appoximation. The Swiss mathematician Leonhard Euler (1707-1783) adopted the lower case Greek letter pi for this ratio in 1737; although he was not the first to use it, he popularized it through his many mathematical writings. Pi sometimes appears to be a unit of angle measure equal to pi radians or 180°. This is because mathematicians regard angle measurement as dimensionless (length divided by length) and therefore omit the unit "radians".
pic: see pik, below.
pica (pi): a unit of length used by typographers and printers. One pica equals 12 points, 4.22 mm, or a bit less than 1/6 inch. "Pica type" is type that sets six lines to the inch.
pico- (p-): a metric prefix denoting 10^-12 (one trillionth). The prefix, pronounced "peek-o" in English, is coined from the Italian piccolo, small. See Prefix
picocurie (pCi): a common unit of radioactivity, used to measure radioactivity occurring naturally in the environment. One picocurie equals 10^-12curie or 0.037 becquerel; this corresponds to one atomic disintegration about every 27 seconds--a very low rate of activity.
picogram (pg): a metric unit of mass equal to 10^-12 gram or one millionth of a microgram.
picoliter (pl or pL): a metric unit of volume equal to 10^-12 liter or 1000 cubic micrometers.
picosecond (ps): a unit of time equal to 10^-12 second.
picul: a unit of weight comparable to the European quintal, widely used in East Asia during the colonial period. The picul is equal to 100 catties, typically about 133.3 pounds or 60.5 kilograms. In recent years the picul has been used as a metric unit equal to 60 kilograms (132.28 pounds) in Thailand or 50 kilograms (110.23 pounds) in China. The unit is pronounced "pickle."
pie: the traditional foot of Italy and Spain. The Italian pie, no longer used, was about 29.8 centimeters. The Spanish pie equals 1/3 vara or 12 pulgadas (see below). The pie used in Spain is about 27.86 centimeters or 10.97 inches, but in Spanish Latin America the pie is generally longer. The Argentine length is 28.89 centimeters or 11.37 inches; in Texas, 1/3 vara is 11 1/9 inches or 28.22 centimeters.
piece (pc): a unit of quantity, equal to 1. This unit, like count (ct) is used to indicate that a measurement represents an exact count of items.
pied: the traditional French foot. Pieds of various lengths were used in France, but the one best remembered now is the royal foot (pied de roi), called the Paris foot in English and the foot (French measure) in Canadian law. The pied de roi equals about 32.48 centimeters or 12.79 inches; the official Canadian definition is 12.789 inches (32.484 06 centimeters). Today the word pied is sometimes used informally in French as a metric unit equal to 30 centimeters.
pieze (pz): a metric unit of pressure, part of the "metre-tonne-second" system sometimes used by European engineers. The pieze is a pressure of one sthene per square meter, or 1000 newtons per square meter, or one kilopascal. This is not such a large pressure; one pieze equals 10 millibars or about 0.145 pound per square inch. The name of the unit comes from the Greek piezein, to press. The unit, spelled pièze in French, is pronounced "pee-ezz" in English.
pik or pic: a traditional unit of distance in the Eastern Mediterranean and Near East. The pik varied considerably but a typical value is about 28 inches (71 centimeters). This is an "arm" unit, like the Italian braccio and the Russian arshin.
pin: a traditional British unit of volume, used for beer. A pin is very different from a pint: it is equal to 1/8 barrel or 4.5 imperial gallons (20.457 liters). There are 2 pins in a firkin.
pinch: an informal unit of volume used in food recipes. In the U.S., the pinch is usually defined to equal 1/8 teaspoon, which is 1/48 fluid ounce or about 0.6 milliliter.
ping: a traditional unit of area in Taiwan, equal to about 3.305 square meters (3.953 square yards). This is the same unit known in Korea as the pyong (see below).
pint (pt) [1]: a traditional unit of volume equal to 1/2 quart. There are three different quarts in use in Britain and the United States, and hence there are three different pints: [i] the U. S. liquid pint, equal to exactly 28.875 cubic inches, 16 fluid ounces, or approximately 473.176 milliliters; [ii] the U. S. dry pint, equal to 33.600 cubic inches or approximately 550.611 milliliters; and [iii] the British imperial pint, equal to 34.678 cubic inches or approximately 568.261 milliliters. In Scotland, the pint is a much larger unit equal to 4 mutchkins, 103.404 cubic inches, 1.491 British imperial quart or 1.695 liter. The origin of the word pint is unclear. See Volume
pint (pt) [2]: a unit of volume used in Australian pubs. A pint of beer is generally 425 milliliters, or roughly 3/4 imperial pint.
pipa: a traditional Portuguese unit of liquid volume, originally very similar in size to the English pipe (see next entry). The pipa has become a metric unit equal to exactly 500 liters, which is 0.5 cubic meter, 132.085 U.S. gallons, or 109.996 British imperial gallons.
pipe: like the butt, the pipe is a traditional unit of liquid volume generally equal to 2 hogsheads. In the U.S., this means a pipe equals 126 U.S.gallons, about 16.844 cubic feet or 476.96 liters. In Britain, things are more complicated, because traditional British hogsheads were of different sizes depending on what they contained. The British pipe was usually used as a wine measure, but even different types of wine had different size pipes.
pixel: Screen resolutions are frequently specified in dpi (dots per inch), which is the reciprocal value of the pixel size multiplied by 25.4 mm. See Typography
planck: an MKS unit of "action" (energy expended over time) or of angular momentum. The planck is equal to 1 joule second or about 0.7375 foot pound second. The unit honors the German physicist Max Planck (1858-1947), the originator of quantum theory.
- plet: a suffix used to create units of quantity. Thus triplet, for example, is a unit of 3. However, words having this ending are also used to mean one member of the group rather than the group as a whole.
plf [1]: symbol for pounds per linear foot (lbm/ft), a common unit of load in engineering. 1 plf = 1.488 164 kilograms per meter (kg/m). The symbol is also used sometimes for pounds of force per linear foot (lbf/ft), in which case 1 plf = 14.593 90 newtons per meter (N/m).
plf [2]: a common abbreviation for "per linear foot." In this use, 1 plf = 3.280 840 per meter (/m).
pli: a common abbreviation for pounds per linear inch. For load, 1 pli = 17.857 97 kg/m. For pounds of force per linear inch, 1 pli = 175.1268 N/m.
-ply: an ending used to indicate the number of folds or layers in an object; thus 4-ply means having 4 layers. The suffix comes from the Latin plicare, to fold.
PM or pm: abbreviation for the Latin post meridiem, "after noon," used after a time to indicate that the time occurs after 12:00 noon. Thus 4:30 pm is the same as 16:30. The notations "AM" and "PM" are used extensively in the United States, where time is usually not stated on a 24-hour basis.
PMPO: abbreviation for "peak music power output," which is often claimed by electronics manufacturers as a unit measuring the effective power output of amplifiers, stereo systems, etc. Buyer beware! There is no industry standard for this "unit," so it is impossible to determine just what it means.
PNC: an abbreviation for preferred noise criterion, a unit used in engineering to measure the level of background noise in rooms or other enclosed spaces. Introduced in 1971, the unit is similar to the older noise criterion (NC), but a PNC rating requires lower levels of high and low frequencies than the corresponding NC rating. PNC ratings below 40 are generally required for residential or classroom spaces. PNC ratings are typically 10-15% lower than raw measurements of the sound level in decibels.
point (pt) [1]: a unit of angle measure equal to 1/32 of a circle. Compasses are read by suspending the compass needle over a compass card traditionally inscribed with a 32-point star. Each point of the star represents a named direction; for example, the first five points from north towards east are labeled North, North by East, North Northeast, Northeast by North, and Northeast. The difference between two directions can be expressed as a certain number of these compass points. One point equals 11°15' of arc or pi/16 radians.
point (pt) [2]: a unit of length used by typographers and printers. When printing was done from hand-set metal type, one point represented the smallest element of type that could be handled, roughly 1/64 inch. Eventually, the point was standardized in Britain and America as exactly 0.013 837 inch, which is about 0.35 mm (351.46 micrometers) and a little bit less than 1/72 inch. In continental Europe, typographers traditionally used a slightly larger point of 0.014 83 inch (about 1/72 pouce, 0.377 mm, or roughly 1/67 English inch), called a Didot point after the French typographer Firmin Didot (1764-1836). In the U.S., Adobe software defines the point to be exactly 1/72 inch (0.013 888 9 inch or 0.352 777 8 millimeters) and TeX software uses a slightly smaller point of 0.351 459 8035 mm. The German standards agency DIN has proposed that all these units be replaced by multiples of 0.25 millimeters (1/101.6 inch). See Typograghy
point (pt) [3]: a unit of proportion, equal to 0.01 or 1%, often called a percentage point.
point (pt) [4]: a unit of mass used for precious stones such as diamonds. One point equals 0.01 carat, or exactly 2 milligrams.
point (pt) [5]: a unit of quantity equal to 1. This unit is used to express changes in an arbitrary score or index, such as the score in an athletic contest. In finance, a change of one point in the Dow Jones average or similar indices represents a change of 1.00 in the index.
point (pt) [6]: a unit used to represent the smallest significant change in an arbitrary ratio. This usage is common in sports. Most sports "averages" are actually ratios of successful performances divided by attempted performances; baseball's batting average is a good example. These ratios are computed to a fixed number of decimal places, usually three, and a point represents a change of 1 in the last decimal place. Thus the batting averages .314 and .302 are said to differ by 12 points.
point (pt) [7]: another name for a mil [1], a unit of distance equal to 0.001 inch (25.4 micrometers). Points are used with this meaning to measure the thickness of card stock in the paper industry.
point (pt) [8]: a measure of the specific gravity of a liquid, typically used in brewing and winemaking. Specific gravity is the mass of a sample of the liquid divided by the mass of an equal volume of pure water. It is a dimensionless (unit-less) number, typically a little larger than 1. Each "point" represents an increase of 0.001 above 1. For example, a liquid of specific gravity 1.048 is described as 48 point.
poise (P, Ps, or Po): a CGS unit of dynamic viscosity. Viscosity is a frictional property (actually, several related properties) of liquids and gases: due to friction between molecules, the liquid or gas resists flowing to a greater or lesser extent. Dynamic viscosity is measured by stating the force needed to move a standard area of one layer of the liquid or gas with respect to another layer, the two layers being parallel and separated by a standard distance filled with the same liquid or gas. If a force of one dyne is needed to move one square centimeter of the liquid or gas relative to a second layer one centimeter away at a speed of one centimeter per second, then the viscosity is one poise. The unit is named for the French physician Jean Louis Marie Poiseuille (1799-1869). Generally speaking, liquids have viscosities measured in centipoises and gases have viscosities measured in micropoises. The SI recognizes no named unit for dynamic viscosity; in SI units, one poise equals 0.1 pascal second (Pa·s). The poise is also equivalent to 14.5038 x 10^-6reyn. P is the proper symbol for the unit, but Ps and Po are also used. See Dynamic Viscosity
poiseuille (Pl): an MKS unit of dynamic viscosity equal to 1 pascal second or 10 poise or 1000 centipoise (cP). See previous entry for more detail.
pole [1]: another name for a rod.
pole [2]: see unit magnetic pole.
polegada: the Portuguese "inch" unit, equal to 1/12 pé, 2.777 centimeters, or 1.093 inches.
poncelet: a unit of power equal to 100 watts, formerly used in France but now obsolete. It was named for a French scientist, J.V. Poncelet (1788-1867).
pond [1]: the Dutch pound, now reinterpreted as a metric unit equal to exactly 500 grams (1.1023 pounds), like the German pfund.
pond (p) [2]: a unit of force formerly used in metric countries. A pond is the gravitational force on a mass of one gram; thus it is equal to 980.665 dynes or 0.002 204 622 6 pounds of force. The kilopond was used more often than the pond.
pony: a small glass for liquor. In the U.S., a pony generally holds exactly 1 (U.S.) fluid ounce or about 29.6 milliliters.
pood: see pud.
pot [1]: a traditional unit of volume in many countries of Europe, roughly comparable to the liter or to the English quart. In Switzerland, the pot is now a metric unit equal to 1.5 liters. In Belgium, the pot is interpreted as 1.5 liters for dry quantities, but only 0.5 liters for liquids. The traditional pot is equal to 0.967 liters in Denmark and to 0.965 liters in Norway.
pot [2]: a unit of volume used in Australian pubs. A pot of beer is 285 milliliters in Queensland and Victoria, 425 milliliters in Western Australia.
pottle: a traditional unit of volume equal to 1/2 gallon. The unit's name is from the French potel, a type of bottle.
pouce: the French "inch" unit, equal to 1/12 pied (see above). Based on the pied de roi, the pouce equals about 1.066 inches or 2.707 centimeters. The word pouce means "thumb" in French.
poumar: an acronym for one pound per million yards, a unit of yarn density formerly used in the U.S. textile industry. One poumar equals about 0.496 055 tex.
pound (lb lbm or #) [1]: a traditional unit of mass or weight. The Romans used a pound (the librapondo, "pound of weight") divided into 12 ounces. All the countries of western Europe used similar units, divided into 12 or 16 ounces, until the advent of the metric system. 12-ounce pounds were common in Italy and southern France, but in Spain and northern Europe 16-ounce pounds became the norm. The word libra is used for this unit in Italy, Spain, and Portugal; in France it is called the livre. Further north, the Latin word pondo ("weight") is the origin of the names of the English pound, Dutch pond, Danish pund, German pfund, and Russian funt. In England, two different "pound" units became standard. The unit now in general use in the United States is the avoirdupois pound, so-called from a French phrase avoir du poids, literally "goods of weight," indicating simply that the goods were being sold by weight rather than by volume or by the piece. The avoirdupois pound is divided into 16 ounces. By international agreement, one avoirdupois pound is equal to exactly 453.592 37 grams; this is exactly 175/144 = 1.215 28 troy pounds. The avoirdupois pound is sometimes abbreviated lb. av. or lb. ap. to distinguish it from the less common troy pound. The symbol lbm is used to distinguish the pound of mass from the pound of force (lbf): see pound force, below.
pound (lb t or lb or #) [2]: a second traditional unit of mass or weight. The troy pound, named for the French market town of Troyes, was the unit used in England by apothecaries and jewelers. The troy pound is divided into 12 ounces, like the Roman pound. One troy pound is 373.242 grams, or exactly 144/175 = 0.822 858 avoirdupois pounds (13.165 72 avoirdupois ounces). The troy and avoirdupois pounds are connected by the grain: there are 5760 grains in a troy pound and 7000 grains in an avoirdupois pound. The troy pound should be abbreviated lb. t. to distinguish it from the more common avoirdupois pound.
pound (lbf or lb) [3]: a traditional unit of force; see pound force, below.
pound (lb) [4]: a traditional unit measuring the weight of paper; see pound weight, below.
poundal (pdl or pl): an English unit of force used in engineering. Since traditional measuring systems, including the English system, did not distinguish between force and mass units, nineteenth century engineers invented the poundal to provide a unit which clearly measures force rather than mass. One poundal is the force which accelerates a mass of one pound at the rate of one foot per second. Since the acceleration of gravity averages about 32.174 ft/sec² at the Earth's surface, one poundal is about 1/32.174 = 0.031 081 pound of force. One poundal is also equal to approximately 0.138 26 newton, or 13 826 dynes. The newton, the SI unit, is now the preferred unit of force in engineering and technical work. See Energy
pound cut (lb cut): a traditional unit of concentration for shellac in the U.S. One pound cut means that the shellac was manufactured by dissolving one pound of dry, bleached shellac in one gallon of alcohol solvent (about 120 grams of shellac per liter of solvent). The most common concentrations sold are 3, 4, and 5 lb cut, but diluted solutions of 1/4 to 1 lb cut are sometimes used as sealers or polishes.
pound foot (lbf ft or lb ft): a traditional unit of torque. Torque is the tendency of a force to cause a rotation; it is the product of the force and the distance from the center of rotation to the point where the force is applied. Thus it can be measured in pounds of force times feet of distance. One pound foot is equal to approximately 1.355 82 newton meter (N m) in SI units. Torque has the same units as work or energy, but it is a different physical concept; to stress the difference, scientists and engineers measure torque in pound feet (or newton meters) and work or energy in foot pounds (or joules).
pound force (lbf or lb): a traditional unit of force. Traditional measuring systems did not distinguish between force and mass units. A force of one pound is simply the gravitational force experienced at the Earth's surface by a mass of one pound. To compute this force, we multiply the mass by the acceleration of gravity, following Newton's law F = ma. Since one pound of mass is 0.453 592 kilograms and the acceleration of gravity averages 9.806 65 meters per second per second at the surface of the Earth, one pound force equals the product of these two numbers, 4.448 221 615 newtons. The symbol lbf should be used for the pound force to distinguish it from the pound of mass.
pound mass (lbm): see pound [1], above.
pound mole (lbmol): a unit of amount of substance. One pound mole of a chemical compound is the same number of pounds as the molecular weight of a molecule of that compound measured in atomic mass units. Thus the pound mole is equal to exactly 453.592 37 moles.
pound per square foot (lbf/ft² or psf): a traditional unit of pressure. 1 psf equals about 47.880 pascals (Pa), 0.478 80 millibars (mb), or 0.192 79 inches of water (in WC).
pound per square inch (lbf/in² or psi): a traditional unit of pressure. 1 psi equals 144 pounds per square foot (psf), 6.894 75 kilopascals (kPa), 68.9475 millibars (mb), 2.036 inches of mercury (inHg), or 27.7612 inches of water (inWC). See below for related notations such as "psig."
pound weight (lb wt or lb): a traditional U.S. unit measuring the weight or thickness of paper. Paper is described as, say, 24 pound weight if one ream (500 sheets) cut in a standard size (called the base size or basis size) has a mass of 24 pounds. For bond paper, the base size is 17 inches by 22 inches (43.18 by 55.88 centimeters), exactly four times the area of an 8.5 inch by 11 inch sheet. This means a ream of 8.5 inch by 11 inch, 24-pound bond paper has a mass of 6 pounds. The metric measure of paper weight is the areal density in grams per square meter (g/m² or "gsm"). 1 lb wt is equivalent to 3.76 g/m² for bond paper, 1.48 g/m² for text stock, 2.70 g/m² for card stock, and varying amounts for the other types of paper.
power (x) [1]: a unit expressing the magnifying power of an optical system. The power is defined to be the angular diameter of the image formed by the system divided by the angular diameter of the original object being observed. In simple telescopes this is equal to the focal length of the primary objective (the big lens or mirror) divided by the focal length of the eyepiece lens. For binoculars, the power is customarily followed by the diameter of the objective lenses, in millimeters, so "8x40" indicates binoculars with a magnifying power of 8 and lenses of diameter 40 mm.
power (x) [2]: similarly, power is used in a variety of other situations to indicate that a measurement is a multiple of some standard quantity. For example, in computer technology, a 16x CD-ROM drive spins a disk 16 times faster than a "standard" speed drive.
ppb, ppm, ppq, ppt, pptr: abbreviations for units of proportion: ppb = part per billion (10^-9), ppm = part per million (10^-6), ppq = part per quadrillion (10^-15), and ppt = part per trillion (10^-12), respectively. However, the abbreviation "ppt" is also used sometimes for part per thousand (10^-3). To avoid this confusion, "pptr" is an alternate abbreviation for part per trillion.
ppcm, ppi: abbreviations for pixels per centimeter and pixels per inch, respectively. A pixel is a single "picture element", so these units measure the resolution, or fineness, of an image.
proof (prf): a traditional unit of proportion used for measuring the strength of distilled liquors, including medicinal solutions of alcohol as well as alcoholic beverages such as whiskey. The proof rating of a liquor is the alcohol content of the liquid expressed as a percentage of the alcohol content of a standard mixture, called the proof liquor. In the United States, the proof liquor is legally defined so to contain exactly 50% alcohol measured by volume. As a result, the U. S. proof rating is equal to exactly twice the percentage of alcohol present, measured by volume. Thus "86 proof" means 43% alcohol. In Britain proof ratings are no longer used, but the former proof liquor contained 57.27% alcohol by volume. This means that 86 proof Scotch, in the U.S., was formerly 75 proof in Britain.
PRU: abbreviation for the peripheral resistance unit, used in physiology and medicine to assess blood flow in the capillaries. A measurement in PRU's is equal to the blood pressure in millimeters of mercury divided by the flow rate in milliliters per minute. That is, 1 PRU equals 1 mmHg·min/mL = 133.3 Pa·min/mL (or, in proper SI units, almost exactly 8 GPa·s/m³).
ps: see pferdestärke above.
psi, psia, psid, psig: traditional symbols for pressure units used in hydraulics and plumbing. psi is a symbol for pound per square inch (see above). psig is a symbol for pound per square inch gauge; this means that the pressure has been read from a gauge which actually measures the difference between the pressure of the fluid and the pressure of the atmosphere. psia means pound per square inch absolute, which is the total pressure including the pressure of the atmosphere. psid, pound per square inch differential, is a symbol for a difference between two pressures, neither of which is atmospheric pressure. Corresponding symbols for pound per square foot (psf, etc.) are also used.
PSU or psu: an abbreviation for practical salinity unit, a standard measure of the salinity of seawater. The "unit" is actually a dimensionless (unitless) ratio obtained by measuring the conductivity of the water sample. Seawater of salinity 35 PSU has the same conductivity as a standard solution of potassium chloride (KCl) with a concentration of 3.243 56 % by mass; a sample of salinity 1 PSU would have conductivity 1/35 that of the standard solution. With this definition, measurements in PSU are very nearly the same as direct measurements of salt ion concentration in parts per thousand.
pu: A unit of distance used during the colonial era in China. The pu equals 5 ch'ih, 5.875 feet, or 1.7907 meters.
pud or pood: a traditional unit of weight in Russia. The pud equals 40 funte or 1/30 packen; this is about 16.381 kilograms or 36.11 pounds. The plural is pudi.
puff (pF): an informal name for the picofarad , a unit of electric capacitance.
pulgada: the traditional Spanish inch, equal to 1/12 pie (see above). The pulgada varies from about 23.2 to 24.1 millimeters (0.913 to 0.949 inch).
puncheon: a traditional unit of liquid volume. The puncheon is often reckoned as equal to 70 gallons. In the U. S. system that would be about 9.358 cubic feet or 264.98 liters; in the British imperial system it would be about 11.238 cubic feet or 308.34 liters. There are other versions of the unit; in one version a puncheon of wine equals 84 wine (or U.S.) gallons (roughly 308 liters); in another, a puncheon of beer equals 72 gallons (roughly 272.5 liters).
pund: the Scandinavian pound, now reinterpreted as a metric unit equal to 500 grams (1.1023 pounds), like the German pfund. The traditional Swedish (Stockholm) pund was equal to about 425.1 grams (14.995 ounces).
pyong: a traditional Korean unit of area equal to about 3.306 square meters or 3.954 square yards. The pyong is widely used in Korea to measure areas both inside and outside buildings. The Taiwanese ping (see above) corresponds closely to the pyong.
pyron: a unit used to measure the heat flow delivered by solar radiation. The pyron is equal to one IT calorie per square centimeter per minute, or 697.633 joules per square meter per second (J/m^2· s). The name is coined from the Greek word pyr for fire.