Electromagnetic radiation with wavelengths shorter than that of ultraviolet radiation and greater than that of gamma radiation. Discovered accidentally by Röntgen in 1895. The primary mechanism for the production of x- rays is deceleration of a rapidly moving charge upon interaction with matter (bremsstrahlung). The x-ray spectrum from an x-ray tube consists of this continuous spectrum on which are superimposed narrow bands (characteristic radiation) that are a consequence of transitions between electronic energy levels of atoms. No sharp boundary exists between x- and ultraviolet radiation nor between x- and gamma radiation, although the latter term is usually restricted to radiation resulting from transitions between nuclear energy levels.