is-pref. Variant of iso-.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.is- combining form variant of iso-: isentropic. Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014is (?z) v. 3rd pers. sing. pres. indic. of be. [before 900; Middle English, Old English; c. Old Norse es, er, Gothic ist, Latin est, Greek estí, Old Church Slavonic jest?, Skt asti] is- var. of iso- before a vowel: isentropic. Is. 1. Isaiah. 2. island. 3. isle. is. 1. island. 2. isle. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.