- any of various machines for grinding, pumping, etc., driven by the force of the wind acting upon a number of vanes or sails.
- (loosely) a wind generator; wind plant.
- Aeronautics. a small air turbine with blades, like those of an airplane propeller, exposed on a moving aircraft and driven by the air, used to operate gasoline pumps, radio apparatus, etc.
- an imaginary opponent, wrong, etc. (in allusion to Cervantes’ Don Quixote): to tilt at windmills.