Plain English Meaning

When the engine quits or is shut down in flight, the air flowing past the propeller can keep it spinning, like wind turning a windmill. The spinning blades create a lot of drag on the airplane.

Full Definition

The condition in which a propeller continues to rotate after engine power is lost, driven by the airflow passing through the propeller disc rather than by the engine. A windmilling propeller produces high aerodynamic drag and, in a multi-engine airplane, significantly degrades single-engine climb performance and controllability.

Context Anchor

Encountered in feathering-propeller discussions, especially after an or shutdown in a multi-engine airplane.