The litre (US liter) (symbol l or L) is a metric unit of volume. The symbol L was introduced in 1979 to avoid confusion with 1 and I. The litre is not an SI unit, but is "accepted for use with the International System" .
It is equal to:
In the past, this was used to define the kilogram, but not anymore, partially because the volume depends ever-so-slightly on the pressure, and pressure units include mass as a factor, introducing a circular dependency in the definition of the kilogram.
The litre is subdivided into smaller units by the application of SI prefixes, making 1 litre equivalent to:
In 1793 the litre was introduced in France as one of the new "Republican Measures", and defined as one cubic decimetre. Its name derived from an older French unit, the litron, whose name came from Greek via Latin.
In 1901, at the 3rd CGPM conference, the litre was redefined as the space occupied by 1 kg of pure water at the temperature of its maximum density (approx. 4 °C) under a pressure of 1 atm. This was supposed to be 1 dm3, but it was later discovered that the original measurement was off, at 1.000 028 dm3.