X-radiation emitted as a result of the retardation of high energy particles by matter.
Source:
PAC, 1991, 63, 735. 'Nomenclature, symbols, units and their usage in spectrochemical analysis - VIII. Nomenclature system for X-ray spectroscopy (Recommendations 1991)' on page 737 (https://doi.org/10.1351/pac199163050735)