/ Dictionary / Index R radioactive fallout: Meaning and Definition of Find definitions for: ra'dioac'tive fall'out. Pronunciation: [key] See under Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. radioactive decay radioactive series Related Content Daily Word Quiz: mamba Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language