noun Physics, Chemistry.
- 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.
- the spontaneous emission of radiation from atomic nuclei. The radiation can consist of alpha, beta, and gamma radiation
1899, from French radioactivité, coined 1898 by the Curies; see radioactive.
- Spontaneous emission of radiation, either directly from unstable atomic nuclei or as a consequence of a nuclear reaction.
- The radiation, including alpha particles, nucleons, electrons, and gamma rays, emitted by a radioactive substance.
- The emission of radiation by unstable atomic nuclei undergoing radioactive decay.
The emission of elementary particles by some atoms when their unstable nuclei disintegrate (see half-life). Materials composed of such atoms are radioactive. (See alpha radiation, beta radiation, and gamma radiation.)