unités

Unités, or units of measurement, are standard quantities used to express physical magnitudes such as length, mass, and time. A system of units provides a common language for science, industry, and daily life by enabling consistent quantification and comparability across contexts and regions.

The International System of Units (SI) is the current dominant framework. It defines seven base units from which other units are derived: metre for length, kilogram for mass, second for time, ampere for electric current, kelvin for thermodynamic temperature, mole for amount of substance, and candela for luminous intensity. In 2019, the SI base units were redefined by fixing exact values for fundamental constants, such as the speed of light for the metre and the Planck constant for the kilogram, to improve stability and universality.

Derived units are formed by combining base units, often with prefixes. Examples include the newton (N) for force, which is kg·m/s²; the joule (J) for energy, which is N·m or kg·m²/s²; and the watt (W) for power, which is J/s. Positive and negative decimal prefixes (such as kilo-, milli-, micro-) express multiples or submultiples of units, for example kilometer (km) or milligram (mg).

In practice, measurements are reported with a value, a unit, and an uncertainty that reflects measurement limitations. Units may be used alongside non-SI units in some domains, but SI units provide a coherent, internationally accepted standard that underpins science, engineering, commerce, and everyday measurement.