What term describes a rotor system being turned by relative wind rather than engine power?

The term that describes a rotor system being turned by relative wind rather than engine power is auto-rotation. This phenomenon occurs when a helicopter experiences a loss of engine power, and the pilot can manage the descent by allowing the rotor blades to spin freely due to the upward flow of air as the helicopter descends. In this state, the rotor blades generate lift, enabling the helicopter to maintain controlled flight and ultimately perform a safe landing.

This process is crucial for pilots to understand, as it demonstrates the ability to utilize aerodynamic principles to maintain control even in emergency situations when engine power is not available. The other options do not pertain directly to the scenario of rotor systems turning due to relative wind; for example, controlled descent refers to a specific flight pattern rather than the mechanics of rotor operation, glide describes a non-powered descent typically related to fixed-wing aircraft, and lift-off pertains to the initial stage of flight when a helicopter is leaving the ground.