NPSH can be defined as two parts:
> NPSH (Available NPSH)
NPSHavailble (NPSHA) is equal to the difference between pump inlet stagnation pressure and vapour pressure head. The absolute pressure at the suction port of the pump.
NPSH required (NPSHR) is the minimum pressure required at the suction port of the pump to keep the pumpfrom cavitating.
NPSHA is a function of your system and must be calculated, whereas NPSHR is a function of the pump and must be provided by the pump manufacturer. NPSHA must be greater than NPSHR for the pump system to operate without cavitation.
NPSHA>NPSHR
Put another way, you must have more suction side pressure available than the pump requires.