Any loss of energy by an excited molecular entity.
See: emission, energy transfer (in photochemistry), internal conversion, radiationless deactivation, radiationless transition, radiative transition
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2235 (https://doi.org/10.1351/pac199668122223)