n. The third letter (Γ, γ = Eng. G) of the Greek alphabet. [ 1913 Webster ]

n. A cross formed of four capital gammas, formerly used as a mysterious ornament on ecclesiastical vestments, etc. See Fylfot. [ 1913 Webster ]

. n. (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, (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. +PJC ]