a building with sails or vanes that turn in the wind and generate power to grind corn into flour.
a structure using wind to generate electricity or draw water.
British a toy consisting of a stick with curved vanes attached that turn in the wind.
(with reference to a person’s arms) move or be moved round in a circle in a manner suggestive of the rotating sails or vanes of a windmill: [with object]:Penny slipped, windmilled her arms, and somehow kept her balance
[no object] (of the propeller or rotor of an aircraft, or the aircraft itself) spin unpowered.
fling (or throw) one's cap over the windmill (s)
dated act recklessly or unconventionally.
windmill in other Oxford dictionaries
Definition of windmill in the US English dictionary