A rotor may refer to:

• the rotating part of a machine, as in a motor, generator, alternator, a pump or a helicopter. The part of the machine that does not rotate is called the stator.
• an electric motor designed to turn a receiving antenna to match the direction of the received signal.
• certain cipher machines, such as the German Enigma machine, made use of rotating wheels, see rotor machine.
• in US English, the disc in a disc brake.
• a part in a distributor.