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).
photocycloadditions are two-step processes involving the formation of diradical intermediates.
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 388
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook