/ Dictionary / Index R radioactive decay: Meaning and Definition of Find definitions for: radioac'tive decay' Pronunciation: [key] ? Physics. Physics. decay (def. 8). Random House Unabridged Dictionary, Copyright 1997, by Random House, Inc., on Infoplease. radioactive dating radioactive fallout Related Content Daily Word Quiz: abdicate Analogy of the Day: Today?s Analogy Spelling Bee: Today?s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language