A gray, abbreviated Gy, is the SI unit of energy for the absorbed dose of radiation. One “gray” is the absorption of one joule of radiation energy by one kilogram of matter. One gray equals 100 rad, an older unit .

It was defined in 1940 by Louis Harold Gray (1905-1965) as “that amount of neutron radiation which produces an increment of energy in unit volume of tissue equal to the increment of energy produced in unit volume of water by one röntgen of radiation”