https://doi.org/10.1351/goldbook.U06561
Particular quantity, defined and adopted by convention, with which other quanitities of the same kind are compared in order to express their magnitudes relative to that quantity. Units have conventionally assigned names and symbols.
Sources:
PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 997 [Terms] [Paper]
VIM [Terms] [Book]