NPSH  (Net Positive Suction Head) : 

The margin of pressure over vapor pressure, at the pump suction nozzle, is Net Positive Suction Head (NPSH).

or, The difference between inlet pressure and the lowest pressure level inside the pump is called NPSH (Net Positive Suction Head). NPSH is therefore an expression of the pressure loss that takes place inside the first part of the pump housing.

In equation form:

NPSH = P_s ‑ P_vap

Where:

NPSH = NPSH available from the system, at the pump inlet, with the pump running

P_s = Stagnation suction pressure, at the pump inlet, with the pump running

NPSH will cause the lowest pressure inside the pump to decrease below the evaporation pressure of the pumped liquid, if the inlet pressure is too low. Consequentially, cavitation occurs in the pump, causing noise and leading to breakdowns.