Amount-of-substance of a component divided by the mass of the system.
The term component is recommended for clinical chemistry, ISO uses "substance".
The term system is recommended for clinical chemistry, ISO uses mixture.
Use of the term level as a synonym for content is deprecated.
In describing a quantity, content must be clearly differentiated from concentration.
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