a quantum of electromagnetic radiation; an elementary particle that is its own antiparticle
"Each photon carries a discrete packet of energy as it travels through space as an elementary particle that is its own antiparticle."
The quantum of light and other electromagnetic energy, regarded as a discrete particle having zero rest mass, no electric charge, and an indefinitely long lifetime. It is a gauge boson.
"The invisible glow emitted by the star was detected by scientists as a stream of photons traveling through space at the speed of light."
In plain English: A photon is a tiny packet of energy that makes up light and other types of radiation.
"Photons are tiny packets of energy that make up all light."
Usage: A photon is the fundamental particle that carries all forms of electromagnetic radiation, including visible light. Use this term when referring to individual packets of energy rather than continuous waves or beams of light.
American physicist Leonard Troland coined "photon" in 1916 to describe light hitting the retina. The word later gained widespread acceptance in modern physics after Gilbert N. Lewis popularized its current meaning of a particle of electromagnetic radiation.