quantum

of radiation
https://doi.org/10.1351/goldbook.Q04986
An elementary particle of electromagnetic energy in the sense of the wave-particle duality.
See: photon
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2267 (https://doi.org/10.1351/pac199668122223)