An effect brought about by electromagnetic radiation the magnitude of which is not proportional to the irradiance. Non-linear optical effects of importance to photochemistry are harmonic frequency generation, lasers, Raman shifting, upconversion, and others.
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2256 (https://doi.org/10.1351/pac199668122223)