The liter is a metric unit of what?

The litre (British English spelling) or liter (American English spelling) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre (m3).

The original French metric system used the litre as a base unit. The word "litre" is derived from an older French unit, the "litron", whose name came from Byzantine Greek—where it was a unit of weight, not volume — via Late Medieval Latin, and which equalled approximately 0.831 litres. The litre was also used in several subsequent versions of the metric system and is accepted for use with the SI (International System of Units), although not an SI unit — the SI unit of volume is the cubic metre (m3).