/ Dictionary / Index M mast cell: Meaning and Definition of Find definitions for: mast' cell" Pronunciation: [key] a large granular cell, common in connective tissue, that produces heparin, histamine, and serotonin. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. mast bed mast clamp Related Content Daily Word Quiz: exemplary Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language