Definition of international unit
: a quantity of a biologically active substance (as a vitamin) that produces a particular biological effect agreed upon as an international standard
First Known Use of international unit
Medical Definition of international unit
: the amount of specific physiological activity of a standardized preparation (as of a vitamin) that is agreed upon as an international standard especially for comparison with other biologicals containing the substance in impure form or with a related biologically active substance; also : the amount of the biologically active substance in the standard amount of the preparation producing this activity
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