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

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

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.