The increase in pressure above the liquid vapor pressure that is necessary for the stable operation of a centrifugal pump. If the pressure in the suction line to a centrifugal pump falls below the NPSH required by the pump, some of the liquid in the suction line will vaporize. This will cause problems for the pump.
The minimum suction head, either required by the pump (NPSHR) or available from the piping system (NPSHA). For proper pump operation (avoiding cavitation), the NPSHA must be greater than the NPSHR. NPSHR is a property of each pump determined by the design of the pump, while NPSHA is a property of the piping layout and elevations of the equipment and the fluid vapor pressure.