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).
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)