Source : Webster's Revised Unabridged Dictionary (1913)

Windmill \Wind"mill`\, n.
   A mill operated by the power of the wind, usually by the
   action of the wind upon oblique vanes or sails which radiate
   from a horizontal shaft. --Chaucer.

Source : WordNet®

windmill
     n 1: a mill that is powered by the wind
     2: generator that extracts usable energy from winds [syn: {aerogenerator},
         {wind generator}]