In general, a ratio of two quantities of the same kind, the numerator quantity applying
to one
constituent (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:
mass fraction,
volume fraction, and
amount fraction (or
mole fraction equal to the
number fraction).
Source:
PAC, 1996, 68, 957
(Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations
1996))
on page 974