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The term SI is the abbreviation for "Système international d’unités", which is the international system of units. This system defines a number of base units that can be combined to create further units. In the context of mechanical and hydrodynamic quantities, the base units are m for length, s for time and kg for mass.

The SI system is based on the International System of Quantities (ISQ), was introduced as long ago as 1960 and is now the world's most widely used system of units for physical quantities.