/ Dictionary / Index I IgM: Meaning and Definition of Find definitions for: IgM Pronunciation: [key] — Immunol. Immunol. immunoglobulin M: a class of short-term circulating and secretory antibodies existing as an aggregate of five antibody molecules, having a high affinity for viruses. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: igm (Thesaurus) igloo ign. Related Content Daily Word Quiz: penury Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language