Buildings or devices with sails or vanes that turn in the wind and generate power. The devices operate by means of a rotating shaft on which sails are mounted or placed at an angle so that the force of wind against them causes rotation, which in turn produced energy. Windmills were historically used chiefly in flat districts for operating a mill to grind grain or pump water; the older and most characteristic European form consists of a conical mill-house with a dome or cap supporting four sails. Modern devices tap the wind to produce electricity by using a disk of sails mounted on a framework.

Source: AAT

http://vocab.getty.edu/page/aat/300006273