The metre, American spelling meter, (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 distance travelled by light in a specific fraction – about one three-hundred millionths – of a second.The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole.