From AMS Glossary
(Or x-radiation, Röntgen ray.) 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.
Boorse, H. A., and L. Motz 1966. The World of the Atom. Vol. 1, . 385–401.