windmill

A windmill is a device that converts moving air into useable energy of some sort (though there are purely ornamental windmills).

This energy can be used to either pump water, make electricity, saw wood or mill grain to give a few examples.

Most modern windmills are refered to with the term wind turbine. Wind power is one of the 'greenest' ways of making energy, and is a good complement to solar power to offset day/night issues and seasonal variations.