Plain English Meaning
A propeller that cannot turn its blades sideways into the wind to stop them spinning if the engine fails. Its blades only move within the normal range used in flight.
Full Definition
A propeller design in which the blades cannot be rotated to an edge-on (feathered) position aligned with the relative wind. The blade pitch range is limited to normal operating angles and does not extend to the streamlined position used to stop a failed engine's propeller from windmilling.
Context Anchor
Seen in propeller system discussions, especially when comparing single-engine and multiengine airplanes after an engine failure.