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]