: a structure that has parts which are turned around by the wind and that is used to produce power, pump water, etc.
Full Definition of WINDMILL
a: a mill or machine operated by the wind usually acting on oblique vanes or sails that radiate from a horizontal shaft; especially: a wind-driven water pump or electric generator
b: the wind-driven wheel of a windmill
: something that resembles or suggests a windmill; especially: a calisthenic exercise that involves alternately lowering each outstretched hand to touch the toes of the opposite foot
[from the episode in Don Quixote by Cervantes in which the hero attacks windmills under the illusion that they are giants]: an imaginary wrong, evil, or opponent —usually used in the phrase to tilt at windmills
See windmill defined for English-language learners