From AMS Glossary
(Or γ-ray, γ ray, γ radiation, gamma radiation.) Electromagnetic radiation originating from transitions between energy levels of atomic nuclei.
A nucleus formed as a consequence of beta or alpha emission sometimes exists briefly in an excited energy level and makes a transition to a lower energy level accompanied by emission of a gamma ray photon with energy equal to the difference between the energies of the initial and final levels. Gamma ray energies from radioactive decay lie in the approximate range 10 keV–6 MeV. Gamma rays are also emitted in nuclear reactions. The boundary between x-rays and gamma rays is fuzzy, the latter term being most often used for electromagnetic radiation of nuclear origin.
Boorse, H. A., and L. Motz 1966. The World of the Atom. Vol. I, . 446–448.