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