Rotor may refer to:
- A rotating part of a mechanical device, for example in an electric motor, generator, alternator or pump.
In engineering:
- Rotor (electric), the non-stationary part of an alternator or electric motor, operating with a stationary element called the stator.
- Helicopter rotor, the rotary wing(s) of a rotorcraft such as a helicopter
- ROTOR, a former radar project in the UK following the Second World War
- Rotor (turbine), the rotor of a turbine powered by fluid pressure
- Rotor (crank), a variable-angle bicycle crank
- Rotor (brake), the disc of a disc brake, in U.S. terminology
- Rotor (brake mechanism), a device that allows the handlebars and fork to revolve indefinitely without tangling the rear brake cable
- Rotor (distributor), a component of the ignition system of an internal combustion engine
- Rotor (engine), the powered part of a pistonless rotary engine
- Rotor (antenna), an electric motor that rotates an antenna to the direction of transmission or reception
In computing:
- Rotor machine, the rotating wheels used in certain cipher machines, such as the German Enigma machine
- Rotor (software project), the former code name for Microsoft's shared source implementation of its Common Language Infrastructure
In mathematics:
- Rotor (mathematics), an n-blade object in geometric algebra, which rotates another n-blade object about a fixed or translated point
- Curl (mathematics), known as rotor in some countries, a vector operator that shows a vector field's rate of rotation
In medicine:
- Rotor syndrome, a rare liver disorder
In biology and chemistry:
- The rotating part of a centrifuge, which also holds the samples
In other fields:
- SC Rotor Volgograd, a Russian football club
- Rotor (Sonic the Hedgehog), a fictional character from the Sonic the Hedgehog universe
- Rotor (ride), the trade name for an amusement ride
- Rotor (meteorology), a turbulent horizontal vortex that forms in the trough of lee waves
- Rotor, a space colony in Isaac Asimov's novel Nemesis
- R.O.T.O.R., a 1989 science fiction/action movie
- Vibrator (sex toy), a Japanese usage of similar sounds in English. Also spelt as rotar.