Definitions for gamma ray
Random House Webster's College Dictionary
a highly penetrating photon of high frequency, usu. 1019 Hz or more, emitted by an atomic nucleus.
a stream of such photons.
Origin of gamma ray:
gamma radiation, gamma ray(noun)
electromagnetic radiation emitted during radioactive decay and having an extremely short wavelength
Very high frequency (and therefore very high energy) electromagnetic radiation emitted as a consequence of radioactivity.