the condition of flight occurring when lift is derived solely from the action of air upon the unpowered rotor of a moving helicopter or autogiro.
Origin of autorotation
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
the continuous rotation of a body in an airflow, such as that of the rotor blades of a helicopter in an unpowered descent
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012