The rate of change, in the case of electrical engineering, is the rate at which the voltage changes over a period of time, and is denoted in volts/second. The rate of change could therefore be called the slew rate.
Where is the period in seconds.
So if there is 40.000 volts discharging through a spark gap in 20ms, the rate of change is
The rate of change can increase while the frequency remains steady, for example by increasing the amplitude or by reducing the duty cycle, in the case of condenser discharges.