The speed at which light travels in a vacuum; 300,000 km per second; a universal constant; SYN. light speed.
Speed at which light and other electromagnetic waves travel through empty space. Its value is 299,792,458 m/186,281 mi per second. The speed of light is the highest speed possible, according to the theory of relativity, and its value is independent of the motion of its source and of the observer. It is impossible to accelerate any material body to this speed because it would require an infinite amount of energy.