Plain English Meaning

When the engine quits or is at very low power in flight, the moving air can keep the propeller spinning on its own. A propeller spinning this way acts like a flat disc in the airflow and creates a lot of drag, which makes the airplane sink faster.

Full Definition

A propeller that continues to rotate after the engine has lost power, driven by the airflow passing through it during flight. The airflow turns the blades like a windmill rather than the engine driving the blades through the air. A windmilling propeller produces significant aerodynamic drag, which steepens the glide and reduces the distance the airplane can travel without power.

Context Anchor

Seen during low-power approaches, power-off approaches, engine-failure practice, and landing discussions where propeller drag affects how steeply the airplane descends.