speed of light

The speed of light (or light speed, or, more precisely, the speed of light in a vacuum, symbolized as c) is a fundamental constant, precisely 299,792,458 meters per second (because the latest refined definition of a meter is c/299,792,458) which is roughly 186,000 miles per second, and is the speed at which any EMR is measured to travel through a vacuum. Due to this less-than-infinite speed, observing at astronomical distances is also looking "backward in time".

Outside a vacuum, light moves slower than this, e.g., in liquid water, about 25% slower. Air has a much smaller effect, but glass (including fiber optics) has the same order-of-magnitude reduction as water, and according to the science of optics, it this reduction that makes glass lenses function.

The speed of light (in a vacuum) always measures to the same quantity, even though light acts like waves. (This is unlike sound waves: if the medium in which sound waves are traveling is also moving relative to you, sound waves passing you in the same direction are moving that much faster.) Relativity spelled out how the speed of light can be constant in this respect, and takes it to be more than merely the speed of EMR: it is the maximum speed at which any influence occurs, such as the effects of gravity, and it is the speed of gravitational waves. Quantum mechanics appears to imply effects that transcend the speed of light, but in very limiting contexts.