Base unit (measurement)
A base unit (also referred to as a fundamental unit) is a unit adopted for measurement of a base quantity. A base quantity is one of a conventionally chosen subset of physical quantities, where no quantity in the subset can be expressed in terms of the others. The SI base units, or Systeme International d'unites, consists of the metre, kilogram, second, ampere, kelvin, mole and candela.
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A base unit is one that has been explicitly so designated; a secondary unit for the same quantity is a derived unit. For example, when used with the International System of Units, the gram is a derived unit, not a base unit.