Intermolecular photochemical process leading to carbo- or hetero- cyclic ring systems by
formation of two new single bonds, either by a
concerted process (e.g., a concerted addition)
or by multi-step processes (e.g., the Paternò–Büchi reaction).
Note:
Triplet state photocycloadditions are two-step processes involving the formation of diradical intermediates.
Source:
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 388