Metre

The metre is the legal (SI) unit of length (symbol: m)
The length of the trajectory in a vacuum travelled by light in 1/299,792,458 of a second

A length equal to 1,650,763.73 times the wavelength in a vacuum of the radiation corresponding to the transition between the 2p10 and 5d5 levels of the krypton 86 atom. ISO standards 1000-1973.