A windmill is a machine that uses wind power to do work, often rotating blades to grind grain, pump water, or generate electricity. It’s a type of engine that converts the wind’s kinetic energy into rotational energy, with sails or blades rotating when the wind blows on them. windmill, device for tapping the energy of the wind by means of sails mounted on a rotating shaft. The sails are mounted at an angle or are given a slight twist so that the force of wind against them is divided into two components, one of which, in the plane of the sails, imparts rotation. Windmills are very useful remote areas where sufficient wind velocity is available.