A discrete quantity of energy that can be thought of as a packet of electromagnetic radiation traveling at the speed of light.
WRCC Climate and Weather Terms Glossary

A quantum (smallest unit in which waves may be emitted or absorbed) of light.
NASA Earth Observatory

Photon - A particle that has no rest mass, no electrical charge and whose energy is determined by its electromagnetic wavelength. A photon is considered to be an amount of electromagnetic radiation energy that is proportional to the frequency of that radiation. It may also be considered as a particle of light with energy, hv, where h is Planck's constant and v is the frequency of light.
[New Scientist; v143; 16-17; 1994.]
[Science; v262; 1008-1009; 1993.]
[Atmospheric Science; J. Wallace and P. Hobbs; page 281; 1977; Academic Press, Inc; New York.]
Atmospheric Chemistry Glossary