/ Dictionary / Index G groundmass: Meaning and Definition of Find definitions for: ground•mass Pronunciation: (ground'mas"), [key] — n. the crystalline, granular, or glassy base or matrix of a porphyritic or other igneous rock, in which the more prominent crystals are embedded. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: groundmass (Thesaurus) ground loop ground meristem Related Content Daily Word Quiz: endear Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language