1) X-radiation 2) X-rays 3) roentgen rays 4) röntgen rays the electromagnetic penetrating radiation having a much shorter wavelength than that of visible light GRM: n. (1), pl.n. (2,3,4) Note: in nuclear reactions it is customary to refer to photons originating in the nucleus as gamma rays and those originating in the extranuclear part of the atom as X-rays References: GENradiation SEEbremsstrahlung