Speed at which electromagnetic radiation propagates in a Space entirely devoid of matter (called also, by way of distinction, absolute vacuum). In a more general sense, a space, as the interior of a closed vessel, which has been evacuated to a high degree by a pump or other artificial means.. Although referred to as the speed of light, this should be more properly called the ‘speed of a massless particle’ as it is the speed at which all particles of zero mass (not only photons, but gravitons and massless neutrinos if they exist) travel in a vacuum. Einstein’s theory of relativity makes several statements about the speed of light:
The speed of light in a vacuum is 299,792,458 m/s (less in a transparent medium such as air, water or glass, depending on the refractive index);
Nothing can travel faster than the speed of light in a vacuum (some particles can exceed the speed of light in a transparent medium – resulting in Cerenkov radiation);
The speed of light in a vacuum is a constant; the speed of light has exactly the same value for observers traveling at different speeds. This property leads to many of the counter-intuitive behaviors predicted by Einstein’s theory of special relativity (e.g. time dilation).