What is the term for the part of the total pressure not used to overcome friction or gravity?

The term for the part of the total pressure not used to overcome friction or gravity is referred to as Residual Pressure.

Residual pressure represents the pressure that remains in a system after accounting for losses due to friction in the hose and elevation changes. It is a critical factor in firefighting, as it helps to ensure that there is sufficient pressure available at the nozzle for effective firefighting operations. When calculating water delivery in firefighting scenarios, it's essential to determine both the static pressure (the pressure when water is not flowing) and the dynamic pressure (the pressure measured when water is flowing) to assess how much usable pressure remains at the nozzle after accounting for losses.

Understanding residual pressure is vital for apparatus engineers and firefighters to ensure that they can deliver adequate water flow rates during fire suppression efforts.