The litre (American spelling: liter; SI symbol l or L) is a non-SI metric system unit of volume equal to 1 cubic decimetre (dm³), or 1,000 cubic centimetres (cm³), or 1/1,000 cubic metre. If the lower case L is used as the symbol, it is sometimes rendered as a cursive ℓ to help distinguish it from the capital "I", although this usage has no official approval by any international bureau.

The word litre is derived from an older French unit, the litron, whose name came from Greek via Latin. The original French metric system used the litre as a base unit, and it has been used in several subsequent versions of the metric system and is accepted for use with the SI, although not an official SI unit — the SI unit of volume is the cubic metre (m³). The spelling of the word used by the International Bureau of Weights and Measures is "litre" and this is also the usual one in most English-speaking countries, but in American English the spelling is "liter", being endorsed by the United States.

One litre of liquid water has a mass of almost exactly one kilogram, due to the gram being defined in 1795 as one cubic centimetre of water at the temperature of melting ice.