Common definition:
The volume of one kilogram of water.
That's however only partially right. 1000cc (cubic centimetres) or one cubic decimeter (equals 0.264172 US gallons) water, as webster says correctly, weighs one kilogram, but only at the temperature of 4° C (39.2º F).
The original definition of a litre (1901) was stated as one kilogram, but this was changed to the current measure of volume.
(Partial info by britannica.com)
also see Litre. The spelling might be an American / British issue, I'm not sure.