The _ _ _ _ _ _ _ _ rate of a changing quantity is the amount it changes per single increment of time (or other quantity).

Change of rate. If the unit or quantity in respect of which something is changing is not specified, usually the rate is per unit of measurement. However, a rate of change can be specified per unit of time, or per unit of length or mass or another quantity. The most common type of rate is "per unit of time", such as speed, heart rate and flux.