a photon is a discrete bundle (or quantum) of electromagnetic (or light) energy. Photons are always in motion and, in a vacuum, have a constant speed of light to all observers, at the vacuum speed of light (more commonly just called the speed of light) of c = 2.998 x 108 m/s.
Like all elementary particles, photons are currently best explained by
quantum mechanics,and exhibit wave particle duality, exhibiting properties of both
wave and particles. The modern concept of the photon was developed gradually by
einstein to explain experimental observations that did not fit the classical wave model of light.