kilogram, abbr. kg, fundamental unit of mass in the metric system, defined as the mass of the International Prototype Kilogram, a platinum-iridium cylinder kept at Sèvres, France, near Paris. Copies of this standard are deposited at bureaus of standards throughout the world, and other units of mass are defined in terms of it. When the metric system was originally devised, the kilogram was defined so that 1,000 cubic centimeters (1 cubic decimeter) of pure water has a mass of exactly 1 kilogram.