means A photon is the smallest possible packet of light or other electromagnetic radiation — the fundamental particle that carries the electromagnetic force.

from Built from Greek phōs, phōtos meaning 'light' — the same root behind photograph and photosynthesis — plus the suffix -on borrowed from electron and ion to mark a particle. The chemist Gilbert Lewis coined the term in the 1920s, though the idea of light coming in discrete 'quanta' was Einstein's two decades earlier; the word arrived to name something physics had already glimpsed.