metre

https://dx.doi.org/10.1351/goldbook.M03884
SI base unit for length (symbol: m). The metre is the length of path travelled by light in vacuum during a time interval of 1 299 792 458 of a second.
Source:
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 981