Kilogram, KIHL uh gram, is the base unit of mass in the International System of Units (SI), the modern metric system. The mass of an object is a measure of its inertia, its resistance to changes in its motion. People also refer to mass as the amount of matter in an object. A bar of platinum and iridium stored at the International Bureau of Weights and Measures in France serves as the SI's standard for a mass equal to exactly 1 kilogram.