From a phenomenological point of view, the term has been used to describe long-lived luminescence. In mechanistic photochemistry, the term designates luminescence involving change in spin multiplicity, typically from triplet to singlet or vice versa. The luminescence from a quartet state to a doublet state is also phosphorescence.
Sources:
PAC, 1984, 56, 231. 'Nomenclature, symbols, units and their usage in spectrochemical analysis-Part VI: molecular luminescence spectroscopy' on page 233 (https://doi.org/10.1351/pac198456020231)
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2258 (https://doi.org/10.1351/pac199668122223)
See also:
Orange Book, 2nd ed., p. 184 (https://media.iupac.org/publications/analytical_compendium/)