| quantity in a conventionally chosen subset of a given system of quantities, where no quantity within the subset can be expressed in terms of the others
NOTE 1 – The subset mentioned in the definition is termed the set of base quantities. The International System of Quantities (ISQ) is defined by a set of seven base quantities.
NOTE 2 – Base quantities are referred to as being mutually independent since a base quantity cannot be expressed as a product of powers of the other base quantities.
NOTE 3 – The quantity “number of entities” can be regarded as a base quantity in any system of quantities.