Definition
A hydraulic pump whose output volume per revolution can be changed automatically while it is running. As system pressure rises toward its design value, an internal control mechanism reduces the amount of fluid the pump moves with each cycle, so the pump delivers only the flow the system actually needs at that moment.
Plain English
A pump that adjusts how much fluid it pushes out on each turn. When the system already has enough pressure, it backs off and moves less fluid; when the system needs more, it moves more.
Context Anchor
Seen in aircraft hydraulic system descriptions, especially systems that power landing gear, brakes, flaps, or flight controls.
Derivation
Displacement here means the volume of fluid the pump moves with each revolution. Variable simply means that volume can change. So a variable-displacement pump is one whose per-revolution output is not fixed.
Why Pilots Care
It supplies only the hydraulic flow required, reducing heat, wear, and power loss compared with fixed-output pumps.
Intuition Check
Do not read “displacement” here as distance moved. In this term, it means the amount of fluid the pump moves each stroke or turn.
Example Sentence 1
The aircraft uses a variable-displacement fluid pump so that hydraulic pressure stays near its design value whether the gear is in motion or fully retracted.
Example Sentence 2
During preflight, the technician verified that the variable-displacement fluid pump maintained steady pressure under varying loads.