unit

A unit is a standard quantity used to express measurements of physical quantities. A numerical value without a unit is meaningless for conveying a measurement, and the unit provides a precise reference for comparison, calculation, and communication.

In the International System of Units (SI), seven base units define all other quantities: the meter for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for amount of substance, and the candela for luminous intensity. Derived units are formed from these base units, such as the newton for force, the joule for energy, and the watt for power. Many common quantities use combinations of base units with SI prefixes (kilo-, milli-, micro-, etc.) to scale measurements up or down.

The SI is standardized by the General Conference on Weights and Measures (CGPM) and implemented worldwide by the International Bureau of Weights and Measures (BIPM). SI prefixes provide a convenient means to express large or small values, and a coherent system aims to relate derived units directly to base units without additional conversion factors.

Not all units are SI; customary or historical units such as foot, inch, liter, or hour continue to be used in various contexts. Unit conversion enables translation between systems, and dimensional analysis uses units to check the consistency of equations. In SI practice, radians and steradians are treated as dimensionless for most calculations, though they are designated as units of angle.

Beyond measurement, the word unit also denotes a single component of a larger system, such as a military unit or housing unit. In measurement contexts, clearly specifying the unit is essential for clarity and accuracy.