gamma ray n.
Electromagnetic radiation emitted from the nucleus of an atom by radioactive decay and having energies in a range from ten thousand (104) to ten million (107) electron volts.
A stream of high-energy electromagnetic radiation given off by an atomic nucleus undergoing radioactive decay. Because the wavelengths of gamma rays are shorter than those of x-rays, gamma rays have greater energy and penetrating power than x-rays. Gamma rays are emitted by pulsars, quasars, and radio galaxies but cannot penetrate the Earth's atmosphere. See more at radioactive decay.