rate of change of a quantity

The time derivative $d Q d t$ 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, $d m d t$ ; rate of change of amount of substance, $d n d t$ .

Source:

PAC, 1992, 64, 1569 (Quantities and units for metabolic processes as a function of time (IUPAC Recommendations 1992)) on page 1572