Definition 1 of 2
Definition
A propeller whose blade angle (pitch) can be adjusted by the pilot in flight, allowing the engine to operate efficiently across a range of airspeeds and power settings. On most modern installations the system is a constant-speed propeller, where the pilot selects a target rpm with the propeller control and a governor automatically varies blade pitch to hold that rpm as flight conditions change.
Plain English
A propeller where the angle of the blades can be changed during flight, instead of being fixed. This lets the pilot fine-tune how the engine and propeller work together for takeoff, climb, and cruise.
Context Anchor
You will see this term in discussions of climb performance, engine controls, and how the propeller setting is adjusted after takeoff or for cruise.
Derivation
The word 'pitch' here refers to the angle of the propeller blades — the same idea as the pitch of a screw thread, which determines how far it advances per turn. A 'controllable-pitch' propeller simply means that angle can be controlled in flight, rather than being set on the ground and left alone.
Why Pilots Care
Proper use improves climb rate, fuel efficiency, and engine longevity by keeping RPM in the optimal range instead of letting it drop or overspeed.
Analogy
It is a little like shifting gears on a bicycle. One setting helps you climb more easily; another setting helps you move efficiently once you are already going faster.
Intuition Check
Pitch does not mean sound here. In this term, pitch means the angle of the propeller blades and how much air they move with each turn.
Example Sentence 1
Because the trainer had a controllable-pitch propeller, the pilot set a low pitch for takeoff and adjusted to a higher pitch once established in cruise.
Example Sentence 2
Once level at cruise altitude the pilot selected coarse pitch on the controllable-pitch propeller to reduce engine RPM and improve fuel economy.