The metre (international 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 1299 792 458 seconds. The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole.