/ Dictionary / Index R radioactivate: Meaning and Definition of Find definitions for: ra•di•o•ac•ti•vate Pronunciation: (rā"dē-ō-ak'tu-vāt"), [key] — -vat•ed, -vat•ing. to make (a substance) radioactive. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. radioactinium radioactive Related Content Daily Word Quiz: modify Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language