gamma ray(n = noun.phenomenon) gamma radiation - electromagnetic radiation emitted during radioactive decay and having an extremely short wavelength;
is a kind of electromagnetic radiation, electromagnetic wave, nonparticulate radiation
is a part of electromagnetic spectrum

   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, (i. e. 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. [Webster 1913 Suppl.]

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