The abbreviation for a Positive Displacement Pump. The reciprocating pumps, metering pumps, and rotary pumps that operate by forcing a fixed volume of fluid from the inlet pressure section of the pump into the discharge zone of the pump. These pumps generally tend to be larger than equal-capacity dynamic pumps. Hydraulic systems frequently use positive displacement pumps at pressures up to 5000 psi (345 bar). A principal advantage of hydraulic power is the high-power density (power per unit weight) that can be achieved. They also provide a fixed displacement per revolution and, within mechanical limitations, infinite pressure to move fluids.