Autorotation refers to:

Autorotation specifically describes a flight condition in which a rotorcraft’s main rotor is driven primarily by the upward flow of air rather than being powered by the engine. In this state, as the helicopter descends, the air flowing up through the rotor system causes the blades to turn and generate lift. This phenomenon allows the pilot to maintain some control over the aircraft's descent and approach for a potential landing, especially in scenarios where engine power is lost.

The concept is crucial for flight safety because it provides pilots with the ability to execute a controlled landing using only the aerodynamic forces acting on the rotor. Understanding autorotation is essential for effective pilot training and emergency procedures, as it can significantly influence the outcome of an engine failure scenario. The other choices describe other aspects of aviation but do not encapsulate the essence of autorotation accurately.