Velocity head is the difference in velocity of the liquid at the suction nozzle and the discharge nozzle. This difference in velocity is caused by difference in the size of the nozzles.If the discharge nozzle is smaller than the suction nozzle, the water will exit at a higher speed at the discharge nozzle than at the suction nozzle.
This difference in velocities creates a head. This head is known as the Velocity Head in Pumps. If the discharge and the suction nozzles are of equal size, the velocity head is zero.