The metre or meterSee American and British English spelling differences(symbol: m) is the fundamental unit of length in the International System of Units (SI). The metre was originally defined by a prototype object meant to represent the distance between the pole and the Equator. Today, it is defined as of a light-second.Because it is the base unit of length in the SI, all SI units which involve length (such as area or speed) are defined relative to the metre. Additionally, due to the metre be