intramolecular photochemical process leading to carbo- or hetero- cyclic ring systems by formation of one new single bond, either by a concerted process (e.g., electrocyclization) or by multi-step processes such as Norrish–Yang reaction, photoinduced electron transfer processes, Yang photocyclization and others.
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 388 (