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.
Green Book, 2nd ed., p. 93 (
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2262 (