/ Dictionary / Index M massless: Meaning and Definition of Find definitions for: mass•less Pronunciation: (mas'lis), [key] — adj. Physics. pertaining to an elementary particle having zero rest mass, as a photon. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. massive retaliation mass man Related Content Daily Word Quiz: soporific Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language