The name given to a small bundle or quantum of electromagnetic energy. It is used when describing the particle-like behavior of electromagnetic waves (including light waves).
Photons travel through empty space at a speed of approximately 186,282 miles (299,792 kilometers) per second (also called lightspeed). This is true no matter what the electromagnetic wavelength. In media other than a vacuum, the speed is reduced. For example, visible light travels more slowly through glass than through outer space.