speed of light (c)
The speed of light in vacuum, written c, is about 299,792,458 metres per second — fast enough to circle the Earth seven and a half times in a single second. It is the speed at which light, radio waves, and every other form of electromagnetic radiation travels through empty space. Since 1983 it has been fixed by definition, which is how the metre itself is now defined in terms of it.
What makes c extraordinary is that every observer measures it to be the same, no matter how they are moving or how the light source is moving. Chase a light beam at almost the speed of light and it still races away from you at the full c, not at the reduced speed common sense would predict. This stubborn constancy is the experimental seed from which all of special relativity grows.
Because of this, c is far more than the velocity of a particular ray; it is the cosmic speed limit for any signal, object, or influence carrying energy or information, and it is the exchange rate between space and time. A light-year of distance and a year of time are, in a deep sense, the same separation measured in different units, with c as the conversion factor that ties space and time into one spacetime.
The same value for every observer, and the link that converts a span of time into a span of space.
Light slows down when passing through glass or water, but that is the speed in a medium, not c. The cosmic speed limit and relativity refer specifically to light's speed in vacuum.