Loss of electronic excitation energy without photon emission or chemical change. Often, ultrafast radiationless decays are mediated by conical intersections.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 410 (https://doi.org/10.1351/pac200779030293)