The spontaneous emission of radiation, generally alpha or beta particles, often accompanied by gamma rays, from the nuclei of an unstable isotope. As a result of this emission the radioactive isotope is converted (or decays) into the isotope of a different (daughter) element which may or may not be unstable. Ultimately, as a result of one or more stages of radioactive decay, a stable (nonradioactive) end product is formed.

See Isotope.