Definition
A cylindrical component that slides inside a sealed cylinder and is moved by pressurized hydraulic fluid to produce mechanical force or motion. In a constant-speed propeller system, hydraulic pistons act on the propeller blades to change blade angle (pitch) in response to pressure regulated by the propeller governor.
Plain English
A piston pushed back and forth by pressurized oil, used to move something mechanical. In a propeller, it's what physically twists the blades to a steeper or shallower angle.
Context Anchor
Seen in propeller blade angle control, especially when describing how oil pressure changes the pitch of a controllable-pitch or constant-speed propeller.
Derivation
Hydraulic comes from the Greek hydor meaning water, originally describing systems that used liquid to transmit force. Piston comes from the Italian pistone, meaning a pestle or pounder -- something that moves up and down inside a tube. Together the term describes a sliding plunger driven by pressurized fluid.
Why Pilots Care
It allows the propeller to maintain the selected RPM automatically by changing blade angle in response to power or airspeed changes.
Analogy
A simple syringe works in a similar way: when fluid pressure moves inside the tube, the plunger moves. A hydraulic piston uses that same basic idea, but in an aircraft system it moves mechanical parts instead of medicine or water.
Intuition Check
Hydraulic does not mean the system uses water. In this propeller context, it usually means pressurized engine oil is being used to move a piston.
Example Sentence 1
When the governor senses the engine is overspeeding, it directs oil pressure to the hydraulic piston, which twists the blades to a coarser angle.
Example Sentence 2
During an engine failure, loss of hydraulic pressure allowed the piston to move the blades to the feathered position.