Definition
A propeller that automatically adjusts the angle (pitch) of its blades to maintain a pilot-selected engine RPM across a range of airspeeds, power settings, and flight attitudes. Blade pitch is controlled by a governor that uses oil pressure to move the blades to a finer pitch (lower angle, higher RPM) or a coarser pitch (higher angle, lower RPM) as conditions change.
Plain English
A propeller whose blades automatically twist to a steeper or shallower angle so the engine keeps spinning at the speed the pilot has set, no matter what the airplane is doing.
Context Anchor
Seen in airplanes with a propeller control, especially during the before-takeoff check when the pilot verifies that the propeller system responds correctly.
Derivation
Constant speed' means the rotational speed (RPM) of the propeller is held constant by the system. The blades change pitch so the speed does not.
Why Pilots Care
It lets the pilot select and hold an RPM that gives best engine performance and fuel efficiency across takeoff, climb, cruise, and descent.
Analogy
Like the gears on a bicycle that automatically shift to keep your pedalling cadence steady -- whether you're going uphill or downhill, your legs spin at the same rate.
Intuition Check
Do not read “constant speed” as constant airplane speed. Here it means the propeller tries to maintain a selected turning speed.
Example Sentence 1
During the before-takeoff check in the constant speed propeller airplane, the pilot cycled the propeller control to verify the governor was responding and warm oil was circulating through the hub.
Example Sentence 2
In cruise the pilot set 2400 RPM and the constant speed propeller automatically adjusted blade angle to hold that speed as the airplane climbed.