Abbreviation for International System of Units
SI (French for Système international d'unités) is the common abbreviation for the internationally most widespread system of units for physical quantities.
The SI is based on the metric unit system, the unit meter is therefore the basic unit. The system is decimal, each derived unit is only a power of base units, without another numerical factor.
The basic sizes are regularly determined by the International Weight and Measures Bureau (BIPM). In the individual countries, these specifications are implemented by national metrological institutes. In Germany, the Physikalisch-Technische Bundesanstalt (PTB) is responsible. Since the SI system is based on metric units, it was never officially introduced in the USA, but is used in some areas.