Amount-of-substance of a component crossing a surface divided by the time. For clinical chemistry the term component is recommended.
ISO uses "substance".
Source:
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 994 (https://doi.org/10.1351/pac199668040957)