A defined and coherent system of units adopted and used by international treaties. (The acronym SI is from the French Systéme International.) SI is international system of measurement for all physical quantities. (Mass, length, amount of substance, time, electric current, thermodynamic temperature, and luminous intensity.) SI units are defined and maintained by the International Bureau of Weights and Measures (BIPM) in Paris, France. The SI system is popularly known as the metric system.