Quantity

Length, extent, diameter, volume, mass, time, population, etc are called quantities.

Length, extent, diameter, etc are physical quantities.

The quantity is a feature that allows the numerical comparison of two equal sizes. A value can be assigned to any quantity.

The term quantity is defined in the DIN 1313: 198-12. Further information in Wikipedia: https://en.wikipedia.org/wiki/Physical_quantity

The term "characteristic" is defined in DIN 55350-12. The characteristic is a feature that can be associated with objective methods, such as measuring a value. The terms characteristic and size have a similar meaning. The postal code is, for example, a characteristic, but no size, because a size comparison or ratio formation of two postal codes does not make sense.

Unit

The value of a size is determined by a numerical value and a unit. Für viele Einheiten sind besondere Einheitenzeichen vereinbart in der DIN 1301. For many units there are special unit symbols, which are agreed in the DIN 1301.

Examples: The units of size "length" are among other meter (m), centimeters (cm). Units of quantity "mass" are e.g. kilogram (kg) or gram (G).

https://en.wikipedia.org/wiki/Units_of_measurement

Dimension

The physical quantities "length" (L), thickness (t), route (s), extent (u) belong to the same dimension "length" with units of meters (m).

The dimension describes the qualitative characteristics of a physical quantity, the quantitative characteristics of a dimension are described with numerical value and unit.

The international SI unit system consists of 7 basic units meter (m), kilogram (kg), second (s), ampere (A), Kelvin (K), mole (mol) and candela (cd) and all the derived units.

The SI unit system is a coherent unit system, because all units may be derived from products of orders of magnitude of the base units.

Unit resolution

Resolutions (Resolutions unit, prefixes) are used to form many multiples or fractions of units, thereby avoiding numbers with pre- or decimal.