Unit of measurement
A defined quantity used as a standard for measuring quantities of the same kind. A measurement expresses another quantity as a number of these units.
Physical quantity: A property that can be quantified, such as length, mass, or temperature. A unit measures a quantity of the same kind.
International System of Units: The internationally agreed system of measurement units, founded on seven base units. It provides the most widely used coherent framework for defining units.
Ancient units of measurement: Length, volume, and mass standards used by ancient societies, often based on bodies or containers. Early units often relied on familiar objects before precise shared standards existed.
Dimensional analysis: A method that uses physical dimensions to test equations and derive relations among quantities. It checks whether equations and unit combinations are physically consistent.
Dimensionless quantity: A quantity whose value is a pure number, with no physical dimension. It needs no unit because it compares quantities of the same kind.
Measurement: The process of assigning a value to a quantity by comparison with a standard. A unit supplies the standard against which a measured quantity is compared.
SI base unit: One of seven SI units assigned to independent base quantities, including the metre and second. Base units anchor the construction of other units in the SI.
Metric system: A decimal system of measurement introduced in France in the late eighteenth century. Its coherent decimal structure helped replace fragmented regional units.
Calibration: The process of comparing an instrument’s readings with standards of known value. Calibration connects practical instrument readings to recognized units.
Scale (measurement): A system for assigning values to observations, including nominal, ordinal, interval, and ratio scales. A measurement scale structures values; it is not itself a unit of quantity.