Radioactive decay is the change of instable atoms to a more stable state. This change to a different nuclide by the spontaneous emission of radiation such as alpha or beta particles, gamma rays, or by electron capture follows an element-specific decay chain. Each step in the decay chain has a definite half-life.
Sometimes also the reduction of excitation energy of the nucleus by e.g. internal conversion is mentioned as radioactive decay.

See also Decay Chain, Radioisotope.