What does kilogram mean? kilogram. Noun 1. In the International System of Units, the base unit of mass; conceived of as the mass of one liter of water, and now defined as the mass of a specific cylinder of platinum-iridium alloy kept at the International Bureau of Weights and Measures in France. Symbol: kg 2. (proscribed) ; Hence, the unit of weight such that a one-kilogram mass is also a one-kilogram weight.