ra·di·o·ac·tiv·i·ty 
(rā′dē-ō-ăk-tĭvĭ-tē)
(rā′dē-ō-ăk-tĭvĭ-tē)n.
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.