What defines a variable displacement pump?

The characteristic that defines a variable displacement pump is its ability to change output flow based on system demand. This type of pump adjusts its displacement, allowing it to increase or decrease the volume of fluid it can move, depending on the requirements of the hydraulic system. This adaptability enhances efficiency, as the pump can meet varying loads without wasting energy on excess flow when it's not needed.

In contrast, a pump that maintains a constant output flow does not adjust to changes in system demand, which can lead to inefficiencies, particularly in systems where the load fluctuates. Additionally, while some pumps are used in low-pressure applications, this is not a defining feature of variable displacement pumps. Furthermore, variable displacement pumps are used in hydraulic systems, not exclusively in pneumatic systems, which involve compressed air rather than fluids.