The metre, symbol \(\text{m}\), is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in a vacuum c to be \(299\ 792\ 458\) when expressed in the unit \(\text{m s}^{-1}\), where the second is defined in terms of the caesium frequency Δν
Cs.
Source:
BIPM, The International System of Units, SI Brochure, 9th ed. (2019), p. 131 (https://www.bipm.org/en/publications/si-brochure/)