The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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