System of quantities , set of quantities together with a set of non-contradictory equations relating those quantities.
In practice, it is useful to identify a set of quantities from which all other quantities can be derived. Such a set is a system of quantities.
In any system of quantities the base quantities that constitute the set are, by definition, considered to be mutually independent – they cannot be described as a product of other base quantities.
The choice of these quantities is by convention. Other choices are equally valid, provided that they satisfy the definition.
However, a specific system of quantities has been agreed and adopted. The Metre Convention established a permanent organisational structure for member governments to act in common accord on all matters relating to units of measurement. It led to the creation of the International Bureau of Weights and Measures (BIPM). The seven base quantities, which are agreed and defined by the General Conference on Weights and Measures (CGPM), are shown in Table 1. This is called the International System of Quantities.
ref: Terminology in Analytica Measurement Introduction to VIM 3
