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).
Sources:
PAC, 1986, 58, 1405. 'Recommendations for the presentation of thermodynamic and related data in biology (Recommendations 1985)' on page 1406 (https://doi.org/10.1351/pac198658101405)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 963 (https://doi.org/10.1351/pac199668040957)