Quantity

Quantity is a property of an object or phenomenon that can be measured or computed, described by a magnitude and a unit. In science, quantities provide a way to describe physical reality with numbers. A quantity may be a scalar, having only magnitude such as mass, temperature, or time, or a vector, having both magnitude and direction, such as velocity or force. More generally, quantities can be represented as tensors in advanced contexts.

Quantities are associated with units, which give scale to the magnitudes. In the International System of Units (SI), base quantities include length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. Derived quantities, such as speed (length per time), acceleration (length per time squared), force (mass times acceleration), and energy (force times distance), are defined from base quantities through equations. Dimensional analysis uses the dimensions of quantities to check consistency, independent of the specific units used.

A measured quantity consists of a value (the magnitude) and a unit, and often an uncertainty or error estimate. Precision is conveyed by significant figures or confidence intervals. Quantities can also be defined abstractly in mathematics or computing as mappings from a domain to a numeric scale, optionally with associated units.

In practice, expressing a quantity as a numerical value with a unit enables comparison, computation, and communication across scientific disciplines. The concept underpins measurements, equations, and models that describe physical systems.