Webster's Revised Unabridged Dictionary · 1913

(Physics) A very penetrating electromagnetic ray not appreciably deflected by a magnetic or electric field, emitted by radioactive substances. Gamma rays are photons of electromagnetic radiation having a wavelength shorter than that of X-rays, (that is shorter than 0.1 nanometer) and are correspondingly more penetrating than X-rays. In addition to being given off in certain types of radioactive decay, they may be found in cosmic radiation, though they are largely absorbed by the earth's atmosphere. Gamma-ray detectors orbited above the atmosphere have found bursts of gamma radiation, in some cases associated with visually observed supernova explosions, but in most cases from unidentified sources.