The Collaborative International Dictionary of English v.0.48:

Gamma ray \Gam"ma ray\ 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]

WordNet (r) 3.0 (2006):

gamma ray
    n 1: electromagnetic radiation emitted during radioactive decay
         and having an extremely short wavelength [syn: gamma
         radiation, gamma ray]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "gamma ray":
   Becquerel ray, Grenz ray, Roentgen ray, X radiation, X ray,
   actinic ray, actinism, alpha radiation, alpha ray, anode ray,
   atomic beam, atomic ray, beam, beam of light, beta radiation,
   beta ray, canal ray, cathode ray, cosmic radiation, cosmic ray,
   cosmic ray bombardment, electron emission, gamma radiation, gleam,
   infrared ray, infraroentgen ray, invisible radiation, leam,
   nuclear rays, patch, pencil, photon, positive ray, radiation,
   radiorays, ray, ray of light, ribbon, ribbon of light, solar rays,
   streak, stream, stream of light, streamer, ultraviolet ray,
   violet ray