Definitions
for unit, quantity, basic quantities, dimensions, prefixes
Physical Quantity
Length, circumference, diameter, volume, mass, duration, population, etc are called physical quantity..
Length, circumference, diameter, etc. are physical quantities.
A physical quantity is a characteristic that allows the numerical comparison of two identical physical quantities. Each quantity can be assigned a value.
The term quantity is defined in 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 property to which a value can be assigned using objective methods, for example through measurement. The terms characteristic and quantity have similar meanings. The postal code, for example, is a characteristic, but not a quantity, because comparing the quantity or ratioing two postal codes makes no sense.
Unit
The value of a quantity is determined by a numerical value and a unit. For many units, special unit symbols are agreed upon in DIN 1301.
Examples: The units of “length” include meters (m), centimeters (cm). Units of "mass" are, for example, kilograms (kg) or grams (g).
https://en.wikipedia.org/wiki/Unit_of_measurement
Dimension
The physical quantities "length" (l), thickness (t), distance (s), circumference (u) belong to the same dimension "length" with the unit meter (m).
The dimension describes the qualitative properties of a physical quantity, while the numerical value and unit describe the quantitative properties of a dimension.
The international SI system of units consists of 7 base units: meter (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol) and candela (cd) and all units derived therefrom. The SI system of units is a coherent system of units because all units can be derived by products of Zener powers of the base units.
