International System of Units also International System n. A complete, coherent system of units used for scientific work, in which the fundamental quantities are length, time, electric current, temperature, luminous intensity, amount of substance, and mass. The International System of Units unit of length is the meter; the unit of time is the second. 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.Interna?tional Sys?tem of U?nits n. an internationally accepted system of physical units, using the meter, kilogram, second, ampere, kelvin, mole, and candela as the basic units of length, mass, time, electric current, temperature, amount of substance, and luminous intensity. Abbr.: SI [translation of the earlier French name Système Internationale d’Unités] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.