Physical quantity

A property of a physical system that can be expressed as a numerical value and a unit. Examples include mass, length, and temperature.

International System of Units: The internationally agreed system of measurement units, including the metre, kilogram, and second. It standardizes the units used to report physical quantities.
Dimensional analysis: A method that uses physical dimensions to check equations and derive relationships. It tests whether equations combining quantities are dimensionally consistent.
Derived quantity: A physical quantity defined from other quantities through a mathematical relationship. Velocity and force arise by combining more basic quantities.
Dimensionless quantity: A physical quantity whose dimensions cancel, leaving a pure number. It remains a quantity even though its unit expression reduces to one.
Unit of measurement: A defined standard used to express the magnitude of a physical quantity. A quantity’s numerical value depends on the unit chosen to express it.
Unit conversion: The process of expressing a measured quantity in a different unit without changing its value. It changes the numerical representation while preserving the measured quantity.
Base quantity: A quantity chosen as a starting point in a system of physical dimensions. Systems of units build other quantities from a selected set of bases.
Pure number: A mathematical value with no physical unit or measurement meaning. It lacks the physical interpretation that distinguishes a quantity from a number.
Physical dimension: A classification of a physical quantity by its dependence on base quantities such as length and time. Dimensions distinguish kinds of quantities and constrain valid equations.
Quantity calculus: An algebraic framework for manipulating physical quantities together with their units. It formalizes calculations in which values and units must remain attached.

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