gamma ray

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A photon of electromagnetic radiation of very short wavelength, less than about 0.01 nanometer, and very high energy, greater than about 100,000 electron volts. Gamma rays are emitted in the decay of certain radioactive nuclei and in electron-positron annihilation.
• noun A narrow beam of such photons.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Very high frequency (and therefore very high energy) electromagnetic radiation emitted as a consequence of radioactivity.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun electromagnetic radiation emitted during radioactive decay and having an extremely short wavelength