spin conservation rule (Wigner rule)

https://doi.org/10.1351/goldbook.S05862
For both radiative and radiationless transitions, the principle that transitions between terms of the same multiplicity are spin-allowed, while transitions between terms of different multiplicity are spin-forbidden.
Sources:
Orange Book, 2nd ed., p. 185 [Terms]
PAC, 1984, 56, 231. (Nomenclature, symbols, units and their usage in spectrochemical analysis-Part VI: molecular luminescence spectroscopy) on page 234 [Terms] [Paper]
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2277 [Terms] [Paper]