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.

   2. 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.

   3. usual symbol for the enzyme unit.

   4. 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.

   5. see mouse unit