/ Dictionary / Index R radioactivity: Meaning and Definition of Find definitions for: ra•di•o•ac•tiv•i•ty Pronunciation: (rā"dē-ō-ak-tiv'i-tē), [key] — n. Physics, Chem. the phenomenon, exhibited by and being a property of certain elements, of spontaneously emitting radiation resulting from changes in the nuclei of atoms of the element. Also called Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: radioactivity (Thesaurus) radioactive waste radio astronomy 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