General definitions

A magnitude is an attribute of a phenomenon, body or substance that can be distinguished qualitatively and determined quantitatively. The term magnitude can refer to a quantity in a general sense, e.g. length, time, mass, electrical resistance, etc., or to a particular quantity, e.g. the length of a given rod, the electrical resistance of a given sample of wire. Quantities that can be ranked in relation to each other in ascending (or descending) order are called quantities of the same kind.

The unit of measurement is a particular quantity, defined and adopted by convention, to which other quantities of the same kind are compared in order to express them quantitatively in relation to this quantity.