Real Dictionary

windmill
countable noun: A windmill is a building with long pieces of wood on the outside which turn around as the wind blows and provide energy for a machine that crushes grain. A windmill is also a similar structure that is used to convert the power of the wind into electricity

windmill in American English
a mill operated by the wind's rotation of large, oblique sails or vanes radiating from a shaft: used as a source of power for grinding grain, pumping water, generating electricity, etc.
to rotate like a windmill
to rotate or cause to rotate solely under the force of a passing airstream
noun: any of various machines for grinding, pumping, etc., driven by the force of the wind acting upon a number of vanes or sails

windmill in British English
noun: a machine for grinding or pumping driven by a set of adjustable vanes or sails that are caused to turn by the force of the wind