An electronic state occuring as the initial or final state of a process involving the absorption or emission of X-ray radiation. It represents a many-electron state which, in the purely atomic case, has total angular momentum (J = L + S) as a well defined quantum number.
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)