Photochemical process leading to the formation of a cyclobutane from two identical alkenes, i.e., from the photocycloaddition of an electronically excited alkene to a ground-state alkene.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 301 (https://doi.org/10.1351/pac200779030293)