Definition
In a hydraulic system, the reservoir is the container that holds the supply of hydraulic fluid. It feeds fluid to the pump, receives returning fluid from the system, allows air bubbles to separate out, and provides space for fluid to expand as it heats up.
Plain English
It is the tank that stores the hydraulic fluid the system uses. Fluid is drawn from it to do work and flows back into it when the work is done.
Context Anchor
Seen in hydraulic system descriptions, maintenance checks, and discussions of landing gear, brakes, or other aircraft parts that may be powered by hydraulic pressure.
Derivation
From the French réservoir, meaning a place to store or keep something in reserve. The aviation use is the same idea: a holding tank that keeps fluid in reserve until the system needs it.
Why Pilots Care
A properly filled reservoir ensures a continuous supply of fluid to power flight controls, landing gear, and brakes; low levels can cause pump cavitation and loss of hydraulic pressure.
Intuition Check
Do not think of a reservoir as just any container of liquid. In a hydraulic system, it is the specific supply-and-return tank that supports the whole fluid system.
Example Sentence 1
Before flight, the mechanic checked the hydraulic reservoir and topped it off to the full mark.
Example Sentence 2
If the reservoir runs low, the hydraulic pumps may lose pressure and the controls become stiff.