Plain English Meaning
A propeller with blades the pilot can twist to a steeper or shallower angle while flying, so the engine can pull the airplane efficiently whether it needs strong acceleration or smooth high-speed cruise.
Full Definition
An aircraft propeller whose blade angle (pitch) can be adjusted by the pilot in flight, allowing the propeller to be set to different angles to suit different phases of flight such as takeoff, climb, and cruise.
Derivation
‘Pitch’ here comes from the same idea as the pitch of a screw — how far it would advance through a solid in one turn. A steeper blade angle takes a bigger ‘bite’ of air per revolution; a shallower angle takes a smaller bite. ‘Controllable’ simply means the pilot can change that bite during flight rather than being stuck with a fixed setting.
Why Pilots Care
Allows the engine to operate efficiently at different speeds, improving climb performance, cruise speed, and fuel economy.
Analogy
Like the gears on a bicycle: a low gear (shallow pitch) gives quick acceleration from a stop, while a high gear (steep pitch) lets you cover more ground per pedal stroke at cruise speed.
Intuition Check
Do not read “pitch” here as nose-up or nose-down airplane attitude. In this term, pitch means the angle of the propeller blades.
Example Sentence 1
After takeoff, the pilot reduced the propeller pitch control to set climb RPM on the controllable-pitch propeller.
Example Sentence 2
Once level in cruise the pilot increased pitch on the controllable-pitch propeller to reduce engine speed and save fuel.