Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

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

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. Spontaneous emission of ionizing radiation as a consequence of a nuclear reaction, or directly from the breakdown of an unstable nucleus.
  • n. The radiation so emitted; including gamma rays, alpha particles, neutrons, electrons, positrons, etc.

from the GNU version of the Collaborative International Dictionary of English

  • n. a 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 ionizing radiation. The radiation emitted by most radioactive substances is one of three types: alpha rays, beta rays, or gamma rays. Some chemical elements have no stable isotopes, and these are referred to as radioactive elements, and the element itself is said to possess radioactivity.

from The Century Dictionary and Cyclopedia

  • n. The property possessed by certain substances of spontaneously emitting obscure rays of a nature distinct from the ether-waves of ordinary radiation.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. the spontaneous emission of a stream of particles or electromagnetic rays in nuclear decay

Etymologies

radioactive +‎ -ity (Wiktionary)

Examples

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