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 mole fraction (or mole fraction equal to the number fraction).
Sources:
Green Book, 2nd ed., p. 41 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 974 (https://doi.org/10.1351/pac199668040957)