U

u [1]: the SI symbol for the unified atomic mass unit, defined in 1960 and accepted by both chemists and physicists.
u [2]: an incorrect replacement for the Greek letter ľ as a symbol for the micron or micrometer. The correct symbol for this unit is ľm. If the Greek letter is not available, the symbol mcm is acceptable.
U [1]: a commercial unit of thermal conductance (heat flow). The U factor, as it is also called, is the conductance through an insulator as measured in Btu's of energy conducted times inches of thickness per hour of time per square foot of area per °F of temperature difference between the two sides of the material. The U factor is numerically equal to 1 divided by the R value.
U [2]: a unit of distance used to measure the height of audio, video, or computer equipment components and the height of the standard racks in which these components are mounted. 1U is equal to 1.75 inches (44.45 millimeters), so that, for example, a 2U component is 3.5 inches high, and a 22U rack houses a stack of components 38.5 inches high.
U [3]: usual symbol for the enzyme unit.
ua: a symbol for the astronomical unit.
uncia: the Roman ounce, equal to about 27.2875 grams of 0.9625 ounce avoirdupois. There were 12 ounces in a Roman pound, and the word uncia means a 12th part; its name gives us the ounce, of course, and also the inch.
unit: when counting, the word "unit" means "one." For example, if a car dealer expects a shipment of 20 units, that means 20 cars. In addition, the word is used as a shorthand for a variety of named "units," such as the international unit in pharmacology. See Prefix
unit (of blood): a unit of volume for human blood and various blood components or products. A unit of whole blood is 450 milliliters, which is about 0.9510 U.S. pint. For components of blood, one unit is the amount of that substance that would normally be found in one unit of whole blood. The adult human body contains roughly 12 units of whole blood.
unit case: a conventional unit of sales volume in the U.S. soft drink industry. A unit case consists of soft drinks, syrup, powder, or whatever equivalent to 24 eight-ounce servings (6 quarts or about 5.678 liters).
unit (magnetic) pole: a CGS unit measuring the strength of a magnetic pole. A unit magnetic pole repels an identical pole at a distance of one centimeter in a vacuum with a force of one dyne. The unit magnetic pole equals about 125.6637 nanowebers (nWb).
universal time (UT or Z): the correct name for the time system previously called Greenwich Mean Time (GMT): the standard time at longitude 0°. Universal time is five hours later than Eastern Standard Time in the U.S. It is always stated on the 24-hour clock; thus an event which occurs at 1:26:15 pm Eastern Standard Time occurs at 18:26:15 UT (five hours after 13:26:15). Technically, the time shown on clocks is figured from Coordinated Universal Time (UTC), an international system of time measurement regulated by very precise atomic clocks. From time to time (either on June 30 or December 31), UTC is adjusted by the addition of a "leap second". This event, widely reported in the press but poorly understood by the public, keeps UTC within 0.9 seconds of mean solar time at longitude 0° as measured by the Earth's slightly uneven rotation.
unze: a traditional German weight unit, corresponding to the English ounce. The unze equals 1/16 pfund. Although the pfund has been assimilated into the metric system (as 500 grams), the unze is effectively obsolete. It varied in size from about 28 to 35 grams. The word comes directly from the Latin uncia; the plural is unzen.
urna: a Roman unit of volume equal to 4 congii, 24 sextarii, or 1/2 amphora. This is equivalent to about 12.75 liters (3.37 U.S. liquid gallons or 2.80 British Imperial gallons). The English word urn is derived from this unit.
USP unit: a unit used in the United States to measure the mass of a vitamin or drug based on its expected biological effects. For each substance to which this unit applies, the U. S. Food and Drug Administration has determined the biological effect associated with a dose of 1 USP unit. Other quantities of the substance can then be expressed in terms of this standard unit. In most cases, the USP unit is equal to the international unit (IU). "USP" is an abbreviation for the United States Pharmacopeia, a handbook describing the established properties of drugs legal for use in American medicine.