metre

https://doi.org/10.1351/goldbook.M03884
SI base unit for length (symbol: \(\text{m}\)). The metre is the length of path travelled by light in vacuum during a time interval of \(\frac{1}{299\ 792\ 458}\) of a second.
Sources:
Green Book, 2nd ed., p. 70 [Terms] [Book]
PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 981 [Terms] [Paper]