The metre or meter 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 as the distance travelled by light in absolute vacuum in ¹⁄_299,792,458 of a second.

The symbol for metre is m (never capital M). Decimal multiples and submultiples of the metre, such as kilometre (1000 metres) and centimetre (¹⁄₁₀₀ metre), are indicated by adding SI prefixes to metre (see table below).