International System of Units

The International System of Units (SI) is the internationally standardized system of measurement units. It organizes seven base units, derived units, decimal prefixes, and definitions based on fixed physical constants.

Metre: The SI base unit of length, defined by fixing the speed of light in vacuum. It anchors SI measurements of distance and defines many derived units.
SI defining constants: The seven exact constants whose fixed numerical values define the SI units. They establish the present-day definitions of all seven base units.
Metric system: A decimal system of measurement units developed in France during the late eighteenth century. SI developed from the metric system’s decimal structure and unit conventions.
Metrology: The science of measurement, including methods for establishing units, standards, and uncertainty. SI is the shared framework metrology uses to make measurements comparable.
2019 redefinition of SI base units: The 2019 revision that defined all SI units through fixed values of physical constants. It completed the transition away from definitions tied to material objects.
Kilogram: The SI base unit of mass, defined by fixing the numerical value of the Planck constant. Its modern definition exemplifies SI’s shift from physical artefacts to constants.
Derived unit: A unit formed by multiplying or dividing base units according to a physical relationship. SI expresses quantities such as force and energy through combinations of base units.
Metre Convention: An 1875 treaty establishing international cooperation on units and measurement standards. It created the institutional framework behind internationally maintained SI standards.
International System of Quantities: A coherent system of physical quantities and equations associated with SI units. It provides the quantity framework whose dimensions SI units express.
International Prototype of the Kilogram: A platinum-iridium cylinder that served as the international mass standard from 1889 to 2019. Its replacement demonstrates the limits of a unit defined by one physical artefact.

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