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 activity.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.