ism (?z??m)n. Informal A distinctive doctrine, system, or theory: “Formalism, by being an ‘ism,’ kills form by hugging it to death” (Peter Viereck).[From -ism.]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.ism (??z?m) ninformal often derogatory an unspecified doctrine, system, or practiceISM abbreviation for (Astronomy) interstellar medium Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014ism (??z ?m) n. a distinctive doctrine, theory, system, or practice: capitalism, socialism, and other isms. [