One of the quantities that, in a system of quantities, are conventionally accepted as functionally independent of one another. In the SI, these are:
length,
mass,
time,
electric current,
temperature,
amount of substance, and
luminous intensity. All other physical quantities (and units) are regarded as being derived from these base quantities (and
base units).
Source:
PAC, 1996, 68, 957
(Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996))
on page 963
PAC, 1986, 58, 1405
(Recommendations for the presentation of thermodynamic and related data in biology (Recommendations 1985))
on page 1406