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.

Aeronautics . (of a propeller engine or turbojet engine) to rotate or cause to rotate solely under the force of a passing airstream.