A set of levels which have the same @[email protected] and the same value of the quantum numbers for total spin \(S\) and total orbital @[email protected] \(L\).
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 [Terms] [Paper]