Autorotation

Autorotation (aircraft Flight Condition)
The rotation of an aircraft about any of its three axes caused by an aerodynamic force. An aircraft in a spin rotates because of an autorotative force.

Autorotation (helicopter Operation)
Descent of a helicopter without engine power applied to its rotor. An aerodynamic force causes the rotors to spin. If the engine in a helicopter fails, the freewheeling unit between the engine and the rotor allows the rotor to free-wheel, or turn without engine power, and an aerodynamic force, called an autorotative force, turns the rotor. A landing made in a helicopter without the use of engine power is called an autorotative landing. 14 CFR Part 1: A rotorcraft flight condition in which the lifting rotor is driven entirely by action of the air when the rotorcraft is in motion.