https://doi.org/10.1351/goldbook.P04585
Generally used to describe a chemical reaction caused by absorption of ultraviolet, visible or infrared radiation. There are many ground-state reactions, which have photochemical counterparts. Among these are photochemical nitrogen extrusions, photocycloadditions, photodecarbonylations, photodecarboxylations, photoenolizations, photo-Fries rearrangement, photoisomerizations, photooxidations, photorearrangements, photoreductions, photosubstitutions, etc.
Note: Photochemical paths offer the advantage over thermal methods of forming thermodynamically disfavoured products, overcome large activation barriers in a short period of time, and allow reactivity otherwise inaccessible by thermal methods.