# 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;[1] 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.