Any of several precision standards used in measuring physical quantities, such as mass, length, and time (see International System of Units ), and also lighting systems, radiation processes, and pharmacology.
The luminous intensity or candlepower of a light is expressed in candelas. The second is based on the frequency of radiation emitted by cesium-133 atoms. In radioactive decay, the international unit is the number of disintegrations per second in a sample. In pharmacology, the international unit is the quantity of a substance (vitamin, hormone, or toxin) that produces a specified effect when tested according to an internationally accepted procedure.