Ratio of the amount-of-substance of the component to the total amount-of-substance in the system containing the component.
The term substance fraction is recommended for clinical chemistry, IUPAC also uses mole fraction and amount fraction.
For clinical chemistry the term component is recommended, ISO uses "substance".
For clinical chemistry the term system is recommended, ISO uses mixture.
PAC, 1996, 68, 957
(Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996))
on page 994
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook