- u 
- the SI symbol for the unified
atomic mass unit, as defined in 1960 and accepted by both chemists and
- u 
- a common replacement for the Greek letter µ as a symbol
for the micron or micrometer. The correct
symbol for this unit is µm.
- a common replacement for µ- as a symbol for the SI prefix micro- (10-6).
The symbol is frequently seen in combinations such as ug for the microgram
(µg). The use of u- was approved by the International Organization for
Standardization in its standard ISO 2955, issued in 1974. The problem at that
time was that many of the character sets in common use did not include the
Greek letter µ. This is much less of a problem today, so ISO 2955 has
been withdrawn and is no longer in effect. The message from all standards
agencies now is: use µ-, not u-, for micro-.
- U 
- 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
- U 
- a unit of distance used to measure the height of the standard racks in which
audio, video, or computer 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 
- usual symbol for the enzyme unit.
- U 
- German symbol for a turn or revolution (Umdrehung), usually seen
in combinations such as U/min, the German equivalent of the English
symbol rpm for revolutions per minute.
- an alternate symbol for the astronomical unit
- an alternate symbol for the international unit
(IU). In many languages, the two words of the phrase "international unit"
are reversed. In French, for example, the phrase is unité international.
- uld, ulp
- units of data precision used in computer science. The symbol ulp
stands for "unit in the last place," the smallest increment in a variable
that can be recorded internally by the machine. Similarly, uld stands
for "unit in the last digit," a change by 1 in the last (right-most) digit
of data represented decimally.
- um, ums
- symbols sometimes used for the micrometer (micron). The symbol um
is acceptable in situations where the Greek letter mu (µ) is not available
to make the proper symbol µm. However, the "plural" symbol
ums is never acceptable, because the SI prohibits
adding -s to a symbol to form a plural.
- uncia 
- a Latin name for the fraction 1/12, subsequently used in many ways to represent
a twelfth part. The Romans had no mathematical notation for fractions. When
they needed to refer to a fractional part of anything, they would often state
its nearest equivalent in unciae.
- uncia 
- the Roman ounce, equal to about 27.2875
grams or 0.9625 ounce avoirdupois. There were 12 ounces in a Roman pound,
and the word uncia means a 12th part; its name gives us both the
ounce and the inch.
- uncia 
- the Roman inch, equal to about 2.473 centimeters
or 0.9734 English 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.
- unit 
- a shorthand word for a variety of named "units," particularly the international
unit in pharmacology.
- 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. Note: although the unit is defined
by volume, it is actually measured by mass at blood collection centers.
Since the density of human blood is 1.053 kilograms per liter, the mass
of a unit of blood is about 474 grams or 16.7 ounces.
- unit  (of heroin)
- a unit of mass or weight for heroin equal to exactly 700 grams (0.7 kilogram
or about 1.543 pounds). This unit, standard
in the southeast Asian drug trade, is also called the Asian unit in
U.S. drug enforcement.
- unit  (of alcohol)
- a unit of the alcohol content of beverages, used primarily in Britain. One
unit of alcohol represents an alcohol content of 10 milliliters or, by weight,
8 grams. The alcohol content of a liter of a beverage is numerically equal
to the percentage of alcohol by volume, so that, for example, a wine that
is 12% by volume contains 12 units per liter, or 9 units in a standard 750
ml (3/4 liter) bottle.
- unit  (of energy)
- in Britain, another name for the kilowatt hour, which was formerly called
the Board of Trade unit.
- unit call (UC)
- a measure of telecommunications traffic density. The unit call is a dimensionless
"unit" representing a traffic density of 100 call-seconds
per hour, or 1/36 erlang.
- 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) or 12.566 37 maxwells (Mx).
- 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 occurring 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. See day
 for additional information.
- 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.
- 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 Latin word urna was also used
more broadly to mean a jug, giving rise to the English word urn.
- U.S. shipping ton
- the freight ton ; see also shipping
- USP unit
- a unit used in the United States to measure the potency of a vitamin or
drug, that is, 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 a registered trademark of the United
States Pharmacopeial Convention, Inc., a private standards organization
that establishes standards for the pharmaceutical industry.
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October 5, 2004