/ Dictionary / Index A antimatter: Meaning and Definition of Find definitions for: an•ti•mat•ter Pronunciation: (an'tē-mat"ur, an'tī-), [key] — n. Physics. matter composed only of antiparticles, esp. antiprotons, antineutrons, and positrons. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: antimatter (Thesaurus) antimasque antimension Related Content Daily Word Quiz: languid Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language