Particle of zero charge, zero rest mass, spin quantum number 1, energy hν and momentum hν/c(h is the Planck constant, ν the frequency of radiation and c the speed of light), carrier of electromagnetic force.
Sources:
Green Book, 2nd ed., p. 93 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2262 (https://doi.org/10.1351/pac199668122223)