/ Dictionary / Index R radioactive: Meaning and Definition of Find definitions for: ra•di•o•ac•tive Pronunciation: (rā"dē-ō-ak'tiv), [key] — adj. Physics, Chem. of, pertaining to, exhibiting, or caused by radioactivity. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: radioactive (Thesaurus) radioactivate radioactive dating 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