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A unit is a measurement of a quantity that is defined or adopted by tradition or law. Other quantities can be expressed as a multiple of the unit.

In human history, various unit systems were developed and used in different regions and cultures. Currently, the global standard of measurement is the International System of Units (SI), which is a modern form of the metric system. Most countries, except the United States, Myanmar and Liberia, have officially adopted SI. The official unit system used in the United States is called U.S customary units, a system very similar to imperial system of measurement. The imperial system was mostly used in the British Commonwealth and the former British Empire. Both U.S customary units and the imperial system were derived from preceding systems used by the English.

Different systems of units have different choices of base units. Base units (or fundamental units) are subsets of units where no subset quantity can be expressed in terms of the others. In the International System (SI) of Units, there are seven base units: kilogram, metre, candela, second, ampere, kelvin, and mole.