In a system of units, that minimal set of units from which all the other units can be derived algebraically. For the old ( pre-2019) SI, the base units were the meter, the kilogram, the second, the ampere, the kelvin, the mole and the candela.

For example, from the kilogram (a unit of mass), the meter (a unit of length), and the second (a unit of time), one can make a unit of force by saying one unit of force is the amount of force that, applied to a mass of 1 kilogram, gives it an acceleration of 1 meter per second per second. That is the definition in SI for the newton.

Before the twentieth century the sizes of base units (with the exception of the unit of time) were represented by physical objects called prototypes, such as the iron bar whose length was the length of the yard in the seventeenth century. Prototypes have gradually been replaced by definitions that rely on reproducible physical phenomena (see, for example, the history of the meter). The last prototype still in active use was the International Prototype of the Kilogram.

Although all the major systems of units have used mass, time and length as base units, that is in part for historical reasons. It is possible to define systems of units in which the units of mass, time and length are not base units, but are derived from other units. Most such systems have been based on atomic constants. It is even possible to have a system of units in which there are no base units.

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Last revised: 8 April 2007.