<Table> <Tr> <Td> </Td> <Td> Look up metre in Wiktionary, the free dictionary . </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> Look up metre in Wiktionary, the free dictionary . </Td> </Tr> <P> The metre (British spelling) or meter (American spelling) (from the Greek noun μέτρον, "measure") is the base unit of length in the International System of Units (SI). The SI unit symbol is m . The metre is defined as the length of the path travelled by light in a vacuum in 1 / 299 792 458 seconds . </P> <P> The metre was originally defined in 1793 as one ten - millionth of the distance from the equator to the North Pole . In 1799, it was redefined in terms of a prototype metre bar (the actual bar used was changed in 1889). In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton - 86 . In 1983, the current definition was adopted . </P>

The si unit of length is the meter which is defined as the length equal to