NPSH refers to net positive suction head. NPSH is particularly relevant inside centrifugal pumps and turbines, which are parts of a hydraulic system that are most vulnerable to cavitation. It may refer to one of two quantities in the analysis of cavitation.
NPSHa: NPSH available at pump suction flange, at given flow rate and temperature
NPSHr: NPSH required at pump suction flange, at given pump speed, flow rate and temperature to prevent cavitation.

Cavitation is a phenomenon that adversely affects the performance of a centrifugal pump and it must be avoided during normal operation. The onset of cavitation in a pump, at any given speed and flow rate, is brought about by a particular combination of temperature and pressure at the pump suction flange. The absolute total head is called the Net Positive Suction Head or NPSH. The letter P tells us that NPSH, by definition, can never be a negative number.