System of units of measurement

A defined set of measurement units and rules for relating them, used to express and compare physical quantities.

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Physical quantity: A property of a phenomenon, body, or substance that can be quantified by measurement. Units attach numerical values to quantities such as length, time, and mass.
Base unit: A unit chosen as a foundation for defining other units in a system. Base units provide the independent starting points for constructing derived units.
Metric system: A decimal system of measurement developed in France in the late eighteenth century. Its decimal structure became the foundation of later international metric systems.
Imperial units: A system of units established in Britain in 1824 and used chiefly in the United Kingdom. Its customary units contrast with the metric system's decimal organization.
Unit of measurement: A defined reference quantity used to express the magnitude of other quantities of the same kind. Every system specifies units that turn measured quantities into numerical values.
Unit conversion: The process of expressing a quantity in a different unit without changing its magnitude. Conversion rules allow measurements in different units to be compared or combined.
French Revolution: A period of political and social upheaval in France beginning in 1789. Revolutionary reformers promoted standardized measures as part of remaking civic life.
United States customary units: A system of units used in the United States and derived largely from English customary measures. Its continued use alongside SI illustrates how unit systems coexist within modern commerce.
Dimensional analysis: The study of physical dimensions and their use in checking equations and relationships among quantities. Dimensions reveal which units can be combined consistently within a system.
Physical constant: A physical quantity whose value is fixed or treated as constant in a specified theory or context. In the modern SI, fixed constants anchor the definitions of several base units.