Gamma radiation, also known as gamma rays, is electromagnetic radiation of high frequency (very short wavelength). They are produced by sub-atomic particle interactions such as electron-positron annihilation, neutral pion decay, fusion, fission or inverse Compton scattering in astrophysical processes. Because gamma rays are a form of ionizing radiation, they pose a health hazard.

A classical gamma ray source is a type of radioactive decay called gamma decay where an excited nucleus emits a gamma ray almost immediately on formation. However, gamma decay may also describe isomeric transition which involves an inhibited gamma decay with a relatively much longer half life.

Gamma rays have frequencies above 10 exahertz (1018 Hz), and therefore have energies above 100 keV and wavelength less than 10 picometers, less than the diameter of an atom. Gamma rays from radioactive decay commonly have energies of a few hundred keV, and almost always less than 10 MeV. There is effectively no lower limit to gamma energy derived from radioactive decay. Energies from astronomical sources can be much higher, ranging over 10 TeV (this is far too large to result from radioactive decay).

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