What is a windmill?

A windmill is a machine with sails or blades, usually four, driven by the wind, originally used to pump water, or grind grain,

Windmills were originally just that, a wind-powered grain mill (to turn grain into flour). Many "windmills", for instance those scattered around the Australian outback, are actually "wind pumps" to bring water from to the surface from deep underground.

Modern ones are used on wind farms that use the renewable source of energy, wind, to make energy.