plasm-pref. Variant of plasmo-.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.plasm- var. of plasmo- before a vowel: plasmapheresis. -plasm a combining form with the meanings ?living substance,? ?tissue,? ?substance of a cell?: cytoplasm; neoplasm. [comb. form representing Greek plásma. See plasma] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.