In general, a ratio of two quantities of the same kind, the numerator quantity applying to one @C01281@ (or part) of the system and the denominator to the sum of quantities for all constituents (parts) of the system. When applied to mixtures fractions represent a group of 3 quantities: @M03722@, @V06643@, and @A00296@ (or @A00296@ equal to the @N04265@).
Green Book, 2nd ed., p. 41 [Terms] [Book]
PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 974 [Terms] [Paper]