Definition of Physical Quantity

A physical quantity is anything that can be measured, and in terms of which the laws of physics are stated -- length, mass, time, force, energy, temperature, electric current, and so on. Every physical quantity, once measured, is expressed as a number together with a unit (e.g. '5 metres'): the number alone, without the unit, is meaningless, since it only tells you the numerical value in whatever unit was chosen, not the physical size of the quantity itself.

Physical quantities are broadly split into two classes: fundamental (or base) quantities -- a small independent set (like length, mass, time) that cannot be expressed in terms of anything simpler -- and derived quantities, which are built up from the fundamental ones by multiplication/division (e.g. speed $=$ length$/$time, force $=$ mass$\times$acceleration). This fundamental/derived split is the basis of the whole system of units and of dimensional analysis co …