The time derivative dQ/dt of the value of the quantity. The differential quotient may also be called derivative (or instantaneous) rate of change. Examples are: rate of change of mass, dm/dt; rate of change of amount of substance, dn/dt.
Source:
PAC, 1992, 64, 1569. 'Quantities and units for metabolic processes as a function of time (IUPAC Recommendations 1992)' on page 1572 (https://doi.org/10.1351/pac199264101569)