gamma ray. A stream of electromagnetic radiation having wavelengths shorter than those of x-rays and therefore greater energy and penetrating power.. 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. 2. A narrow beam of such photons.