One litre (liter) is the volume of one cubic decimetre, that is a cube of 10 × 10 × 10 centimetres. i.e.: a unit of measure that equals to exactly one cubic decimeter (dm3).
It is the volume of 1 kilogram of water and there are 33.814 fluid ounces in the same volume of water.
One litre of water at 4 degrees Celsius has the mass of exactly one kilogram.
In British English it is spelled “Litre“, in American English it is spelled “Liter“.