basic unit of mass in the metric system, equal to the mass of the international prototype of the kilogram, a platinum-iridium cylinder kept at the International Bureau of Weights and Measures laboratory at Sèvres, France. A kilogram is very nearly equal (it was originally intended to be exactly equal) to the mass of 1,000 cubic cm of water. The pound is now defined as equal to 0.45359237 kg, exactly. As originally defined, the kg was represented in the late 18th century as a solid cylinder of platinum. Measurements of the mass of a volume of water proved to be imprecise and inconvenient to make, however, and the platinum artifact itself became the standard. It was superseded in 1889 by the present standard kilogram, also a solid cylinder, of height equal to its diameter, made of the same alloy as the standard metre bar then in use. See also International System of Units.