A type of fluid pump whose output can be changed to suit the demands placed on it. A centrifugal pump moves an amount of fluid determined by the opposition at its discharge port. The less opposition there is, the greater the amount of fluid the pump moves.

Variable-displacement piston pumps control their output by either changing the physical distance the pistons move by using a variable wobble plate, or by changing the length of time the intake ports of the cylinders are open.