Photo-induced substitution resulting from the reaction of radicals produced by photolysis of an appropriate reagent, such as, e.g., halogenated compounds.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 412 (https://doi.org/10.1351/pac200779030293)