| fundamental physical constant the value of which has been fixed at exactly 299 792 458 m/s
Note 1 to entry: Any electromagnetic wave or gravitational wave propagates in vacuum with luminal speed. Luminal speed is also the upper limit for transfer of information.
Note 2 to entry: The commonly used terms "speed of light" and "light speed" are deprecated, because this quantity is also observed in matter where its value is always less than that in vacuum.
Note 3 to entry: Related concepts are "subluminal speed" and "superluminal speed".