/ Dictionary / Index D decay constant: Meaning and Definition of Find definitions for: decay' con"stant Pronunciation: [key] — Physics. Physics. the reciprocal of the decay time. Also called Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. decay decay series 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