basisunits

Basis units, often called base units, are the fundamental quantities of a measurement system from which all other quantities are derived. They form a basis for the unit system, meaning every unit in the system can be expressed as a product of base units raised to integer or fractional powers.

In the International System of Units (SI), there are seven base units: second (s) for time, metre (m) for length, kilogram (kg) for mass, ampere (A) for electric current, kelvin (K) for thermodynamic temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity. These base units are defined by fixing exact values of fundamental constants: the second by the cesium-133 atomic transition, the metre by the speed of light in vacuum, the kilogram by the Planck constant, the ampere by fixing the elementary charge, the kelvin by the Boltzmann constant, the mole by the Avogadro constant, and the candela by a fixed luminous efficacy of radiant power.

Derived units are formed from base units, such as the newton (N = kg·m/s^2), the joule (J = N·m = kg·m^2/s^2), the watt (W = J/s = kg·m^2/s^3), the pascal (Pa = N/m^2), and the hertz (Hz = s^-1).

Base units are chosen to be independent and practically realizable, providing a stable framework for measurement. Different measurement systems may use other sets of base units, but the SI base units are the globally dominant reference in science and engineering.