/ Dictionary / Index A active mass: Meaning and Definition of Find definitions for: ac'tive mass' Pronunciation: [key] — Chem. Chem. the effective concentration of a substance. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. active layer active-matrix Related Content Daily Word Quiz: perennial Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language