international unit

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The quantity of a biologically active substance, such as a hormone or vitamin, required to produce a specific response.
• noun A unit of potency for similarly active substances, based on this quantity and accepted as an international standard.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A particular, minute unit of mass, defined differently for different substances, but so that varying substances of the same general type have the property that one international unit of the one has the same effect on the human body as one international unit of the other.