international unitn. Abbr. IU1. The quantity of a biologically active substance, such as a hormone or vitamin, required to produce a specific response.2. A unit of potency for similarly active substances, based on this quantity and accepted as an international standard.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.in?terna?tional u?nit n. 1. an internationally accepted standard, derived by bioassay, to which samples of a pharmaceutical substance are compared for ascertaining their relative potency. 2. the specific biologically effective quantity of such a substance. Abbr.: IU [1920?25] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.international unit A unit for measuring a biologically active substance, such as a hormone or vitamin.The American Heritage® Student Science Dictionary, Second Edition. Copyright © 2014 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.