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.
An object is called radioactive if it undergoes radioactive decay.
The property possessed by certain nuclides of spontaneously emitting alpha or beta particles and, sometimes, gamma radiation by the disintegration of atomic nuclei. The units for the rate of decay of radioactive nuclides are the curie, Ci (3.7 x 1010 disintegrations per second exactly), or, in SI units, the becquerel, Bq (1 disintegration per second).