Definition of windmill
1 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
2
: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
3
[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 
