/ Dictionary / Index R radioactinium: Meaning and Definition of Find definitions for: ra•di•o•ac•tin•i•um Pronunciation: (r?"d?-?-ak-tin'?-um), [key] ? n. Chem. the radioactive isotope of thorium having a mass number 227 and a half-life of 18.8 days. Symbol: RdAc, Th 227 Random House Unabridged Dictionary, Copyright 1997, by Random House, Inc., on Infoplease. radioacoustics radioactivate Related Content Daily Word Quiz: abscond Analogy of the Day: Today?s Analogy Spelling Bee: Today?s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language