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A rotating-impeller pump that transfers mechanical energy to a fluid. At a fixed speed its head varies with flow; its characteristic curve and the connected system determine the pump operating point.

A pump supplies mechanical energy to a fluid so that it can flow against resistance or gain elevation. For a steady incompressible stream, a rise in pressure $\Delta p$ at similar inlet and outlet heights and speeds represents useful fluid power $P=Q\Delta p$, where $Q$ is the volumetric flow rate. Differences in kinetic energy and gravitational potential energy must be included when those conditions do not hold.

A real installation chooses its flow through the interaction between the pump's pressure-flow response and the connected passages. More resistance generally shifts the pump operating point, so the nameplate or rotational speed alone does not determine mass flow rate. Measuring inlet/outlet pressures, flow and supplied mechanical power separates useful fluid output from dissipative losses. A centrifugal pump uses a rotating impeller; a displacement device instead moves a defined volume during a cycle, with leakage and compressibility affecting the actual delivery.