photon
A photon is a single quantum of light — the smallest indivisible unit of electromagnetic energy. When you dim a beam further and further, you do not get an ever-fainter continuous glow; you eventually get a thin patter of individual photons arriving one at a time, like single raindrops. Each one carries a definite energy, E = hf, set by the light's frequency, along with momentum and a property called polarization.
A photon has no rest mass, and it always travels at the speed of light — it can never be slowed or brought to rest. Because of this it is a thoroughly relativistic object, and its energy and momentum are linked by E = pc. Despite carrying momentum, it pushes on matter only gently; the faint pressure of sunlight on a solar sail is a stream of photons handing over their momentum.
The photon is the cleanest illustration of wave–particle duality. The same light that diffracts and interferes like a wave also arrives, is absorbed, and is counted as discrete particles. A photon is not a tiny billiard ball, and it is not a little classical wave packet either; it is a genuinely quantum entity whose wave aspect governs where it is likely to go and whose particle aspect governs how it is detected, all at once.
Massless, always moving at c, carrying energy set by frequency and momentum by wavelength.
It is tempting but wrong to picture a photon as a tiny localized ball flying along. Until it is detected it has no definite path; its wave nature decides only the probability of where it will land.