The first observable chemical entity which is produced in the primary photochemical process and which is chemically different from the reactant.
See: primary (photo)process
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2266 (https://doi.org/10.1351/pac199668122223)