speed of light ?

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) but 3×108 m/s is commonly used in rough estimates. Note that this speed is a definition, not a measurement. Because the fundamental SI unit of length, the metre, has been defined since October 21, 1983 in terms of the speed of light, one metre is the distance light travels in a vacuum in 1/299,792,458 of a second. Thus, any further increase in the precision of the measurement of the speed of light will actually change the length of the metre; the speed of light will remain precisely 299,792,458 m/s. In imperial units, the speed of light is about 186,282.397 miles per second, that is about one foot per nanosecond.

A line showing the speed of light on a scale model of Earth and the Moon

When passing through a transparent or translucent material medium, like glass or air, light will have a slower speed than in a vacuum; the ratio of c to the observed phase velocity is called the refractive index of the medium. In general relativity, a gravitational potential can affect the speed of distant light in a vacuum, but locally light in a vacuum will always pass an observer at a rate of c.