The speed of light (or light speed, or, more precisely, the speed of light in a vacuum, symbolized as c) is a fundamental constant measuring 299,792,458 meters per second, roughly 186,000 miles per second, and is the speed at which EMR is measured to travel through a vacuum. Due to this less-than-infinite speed, looking astronomical distances also is looking "backward in time".
Outside a vacuum, light moves slower than this, e.g., in liquid water, about 25% slower. Air has little effect, but glass (and fiber optics), for example, 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 the sound waves are traveling is also moving relative to you, the waves are traveling past you that much faster.) Relativity gave details of the manner in which the speed of light can be constant in this respect, and showed it to be more than merely the speed of EMR: it is the speed at which any influence occurs, especially including gravity, for example, being the speed of gravitational waves. Quantum mechanics appears to have effects that transcend it, but in very limiting contexts.