The metre or meter (from the Greek ìÝôñïí /´metron/) [1] is a unit of length. It is the basic unit of length in the metric system and in the International System of Units (SI), used around the world for general and scientific purposes. Historically, the metre was defined by the French Academy of Sciences as the length between two marks on a platinum-iridium bar, which was designed to represent 1⁄10,000,000 of the distance from the equator to the north pole through Paris. In 1983, it was redefined by the International Bureau of Weights and Measures (BIPM) as the distance travelled by light in free space in 1⁄299,792,458 of a second.[2]The BIPM does not distinguish between quantum vacuum and free space.[3]