Gamma decay is the process by which the nucleus of an atom emits a high energy photon, that is, extremely short-wavelength electromagnetic radiation.
Gamma decay is analogous to the emission of light (usually visible light) by decay in the orbits of the electrons surrounding the nucleus. In each case the energy states, and the wavelengths of the emitted radiation, are governed by the laws of quantum mechanics. But while the electron orbits have relatively low energy, the nuclear states have much higher energy. For example, the sodium "D" spectral line has a wavelength of 0.6 microns and a corresponding quantum energy of about 2 electron volts, whereas a gamma ray emitted after cobalt-60 decay has a wavelength of about 1 picometer (10-12 meters) and a quantum energy of about 1 million electron volts.
Nuclei are not normally in excited states, so gamma radiation is typically incidental to alpha or beta decay—the alpha or beta decay leaves the nucleus in an excited state, and gamma decay happens soon afterwards.
Gamma radiation is the most penetrating of the three kinds. Gamma ray photons can travel through several centimeters of aluminum, for example.