A gamma ray, also known as gamma radiation (symbol γ), is a penetrating form of electromagnetic radiation.
It comes from the radioactive decay of atomic nuclei. Gamma rays have the shortest wavelength electromagnetic waves. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium.
In 1903, Ernest Rutherford called this radiation "gamma rays" because they strongly penetrated matter. In 1900 he had already named two less penetrating types of decay radiation (discovered by Henri Becquerel): alpha rays and beta rays in the order of penetrating power. Gamma rays penetrated matter even more strongly.
Gamma rays are ionizing radiation and are hazardous to life. They can damage bone marrow and internal organs. Unlike alpha and beta rays, they easily pass through the body and so they are a formidable radiation protection challenge. They need shielding made from lead or concrete. On Earth, the magnetosphere protects life from most types of lethal cosmic radiation other than gamma rays.