radioactivity
nounSpontaneous emission of radiation, either directly from unstable atomic nuclei or as a consequence of a nuclear reaction.
nounThe radiation, including alpha particles, nucleons, electrons, and gamma rays, emitted by a radioactive substance.
nounThe property possessed by certain substances of spontaneously emitting obscure rays of a nature distinct from the ether-waves of ordinary radiation.
nouna form of instability which is a property of the atomic nuclei of certain isotopes, which causes a spontaneous change in the structure of the nucleus, accompanied by emission of energetic radiation. The radiation emitted is usually sufficient to cause ionization in matter through which it passes, and is therefore called
The radiation so emitted; including
the spontaneous emission of a stream of particles or electromagnetic rays in nuclear decay