1. Spontaneous emission of radiation, either directly from unstable atomic nuclei or as a consequence of a nuclear reaction.
2. The radiation, including alpha particles, nucleons, electrons, and gamma rays, emitted by a radioactive substance.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.