In 1896, just a year after Wilhelm Roentgen had discovered x-rays, the French physicist Henri Becquerel discovered a new kind of ray, quite similar to x-rays, but originating from uranium. He labeled it "metallic phosphorescence", while in actuality what he had observed was gamma radiation (being emitted by radium-226, a part of uranium decay). In 1900 the French chemist and physicist Paul Villard identified gamma radiation while studying the radiation produced by the substance radium. It was not until 1903, however, that Ernest Rutherford named the rays gamma rays, gamma being the third letter in the Greek alphabet, to distinguish it from the alpha and beta rays (thorium* and uranium, respectively) that he [Rutherford] had already discovered.
The original distinction between x-rays and gamma rays was the wavelength. Radiation producing a wavelength below a number such as 10−11 m was labeled as gamma radiation, while radiation producing a wavelength above it was called an x-ray. However, as longer wavelength gamma radiation emitters were found, the distinction begin to overlap. Now they are distinguished primarily by their source; x-rays are emitted by the electron fields outside the nucleus, while gamma rays originate in the nucleus itself. Both gamma rays and x-rays are part of the electromagnetic spectrum.
*Thorium is an element that the Swedish chemist Jons Jakob Berzelius named after Thor, the god of Thunder
