radioactive decay

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Spontaneous disintegration of a radionuclide with the emission of energetic particles or radiation, such as alpha or beta particles or gamma rays.
• noun An instance of such disintegration.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics Any of several processes by which unstable nuclei emit subatomic particles and/or ionizing radiation and disintegrate into one or more smaller nuclei.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the spontaneous disintegration of a radioactive substance along with the emission of ionizing radiation