Gamma completes the alpha-beta-gamma sequence Ernest Rutherford used in 1899 to classify newly discovered radioactive emissions by how far they penetrated matter; unlike the alpha particle, a helium nucleus, and the beta particle, an electron, the gamma ray Rutherford named is not a particle of matter at all but a high-energy photon, a form of electromagnetic radiation more penetrating than either, first identified by the French physicist Paul Villard in 1900. That naming convention, three Greek letters standing for three different physical phenomena discovered within a single decade, has no ancient precedent and shares nothing with the letter's ancient Semitic root gimel, meaning camel, discussed elsewhere in this atlas.
Mathematicians use gamma for an unrelated purpose, the gamma function, a continuous extension of the factorial defined by Leonhard Euler in the 18th century; photographers and display engineers use gamma correction to describe the nonlinear relationship between an image's stored values and its displayed brightness; and, following the Bayer star-naming system of 1603, Gamma conventionally marks the third-brightest star in a constellation. Gamma is also, along with several neighbouring letters, part of the Milesian numeral system covered elsewhere in this atlas, a much older and entirely separate numerical role that predates every one of these 19th- and 20th-century scientific senses by more than two thousand years.