The time derivative \(\frac{\rm{d}Q}{\rm{d}t}\) 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, \(\frac{\rm{d}m}{\rm{d}t}\); rate of change of amount of substance, \(\frac{\rm{d}n}{\rm{d}t}\).