SIdefinities

SIdefinities refers to the formal set of definitions that underpins the International System of Units (SI). It includes precise statements for the seven base units, the derived units obtained from them, and the constants fixed by convention to realize those units in practice. In contemporary usage, the term is often encountered in metrology and science communication, particularly in Dutch-language materials where definities denotes formal definitions and SI denotes the unit system.

Base units cover meter, kilogram, second, ampere, kelvin, mole, and candela. The system also uses derived units such as newton, joule, and watt, defined by algebraic relationships among base units. The SI also includes two supplements, radians and steradians, which are treated as dimensionless derived units in modern definitions.

Since 2019, the SI base units are defined by fixing exact numerical values of fundamental constants: speed of light in vacuum c defines the meter; cesium-133 hyperfine frequency defines the second; Planck constant h defines the kilogram; elementary charge e defines the ampere; Boltzmann constant k defines the kelvin; Avogadro constant NA defines the mole; luminous efficacy defines the candela. This redefinition decouples unit magnitudes from physical artifacts and ties them to invariant constants.

Definitions are published and maintained by the BIPM and are formalized in the SI Brochure, updated by the CGPM on a periodic schedule. In practice, SIdefinities provide unambiguous, universally reproducible standards for measurement and science, ensuring consistency across laboratories and industries.