Definition - What does Radioactive Decay mean?
Radioactive decay is used to describe what happens to the nucleus of an unstable atom when it loses energy and emits radiation. Any material that emits any of the different forms of radiation is deemed radioactive. The term radioactive decay is more commonly known as radioactivity, is also called nuclear decay and involves any material that is considered radioactive.
Safeopedia explains Radioactive Decay
An unstable atom is an atom that does not have enough binding energy to keep the nucleus of the atom together due to an imbalance of protons and neutrons. This is due to an excess of either protons or neutrons. There are several different kinds of radiation that are emitted from an unstable atom, which include alpha particles, beta particles, gamma rays and conversion electrons.