1. A decrease in the water pressure (in psi) which occurs as the water flows. Pressure drop may occur for several reasons: internal friction between the molecules of water, external friction between the water and the walls of the piping system, or rough areas in the channel through which the water flows. 2. The difference between the inlet and outlet water pressure during water flow through a water treatment device such as a water conditioner. Abbreviated P and measured in pounds per square inch gauge pressure. Same as head loss.