UnitsOfMeasure

A Unit of Measure is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of measure(ment).

See
http://www.unitsofmeasurement.org

Wikipedia
http://en.wikipedia.org/wiki/Unified_Code_for_Units_of_Measure
or 
http://en.wikipedia.org/wiki/Units_of_measurement on further background.