radioactinium: Meaning and Definition of

ra•di•o•ac•tin•i•um

Pronunciation: (r?"d?-?-ak-tin'?-um), [key]
? n. Chem.
  1. 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.