noun Physics, Chemistry.

  1. 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.


  1. 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.


  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.

  1. 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.)

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