The amount of substance of a specified component changed in or moved to or from a system divided by the time during which the component was changed or moved. This kind of quantity usually should be preferred to mean mass rate or mean volume rate.
Source:
PAC, 1979, 51, 2451. 'Quantities and Units in Clinical Chemistry' on page 2475 (https://doi.org/10.1351/pac197951122451)