The International System of units (SI units) is the modern refined version of metric system currently in use worldwide. It was introduced in 1960 and is abbreviated SI from French, Le Système International d’Unités. An important feature of the SI units is internationally accepted standardization. Each unit has a universally recognized size. SI units are continuously improved to satisfy the needs in terms of accuracy and practical realization. Formal definitions of all SI base units are derived, adopted and modified time to time by the CGPM.
SI units have seven base units and al other SI units can be derived from these base units. There are numerous derived units that are complementary to the base units. They may have special names (e.g. hertz, pascal, Becquerel, etc.) but can always be expressed in terms of the base units. Derived units are a matter of convenience, as they can be expressed in terms of basic units. Many other units, such as the liter, are formally not part of the SI, but are accepted for use with SI.