The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning "swiftness". It is the speed of all electromagnetic radiation in a vacuum, including visible light, and more generally it is the speed of anything with zero rest mass.
In metric units, c is exactly 299,792,458 metres per second (1,079,252,848.8 km/h). The fundamental SI unit of length, the metre, has been defined since October 21 1983 as the distance light travels in a vacuum in 1/299,792,458 of a second; any increase in the precision of the measurement of the speed of light would refine the definition of the metre, and not alter the numerical value of c. 3×108 m/s is commonly used in rough estimates. In imperial units, the speed of light is about 186,282.397 miles per second, that is about one foot per nanosecond.
The speed of light when it passes through a transparent or translucent material medium, like glass or air, is slower than in a vacuum; the ratio of c to the observed phase velocity is called the refractive index of the medium. General relativity explains how a gravitational potential can affect the apparent speed of distant light in a vacuum, but locally light in a vacuum always passes an observer at a rate of c.