A photophysical process. An isoenergetic
radiationless transition between two electronic states having different multiplicities. It often results in
a vibrationally excited molecular entity in the lower
electronic state, which then usually deactivates to its lowest vibrational level.
Source:
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2248
See also:
Orange Book, p. 187