windmill
A structure whose rotating blades convert wind energy directly into mechanical power for a local application.
A windmill is a structure built to harness wind energy with rotating blades and convert it to mechanical energy for a local application. This is an ancient form of structure, and should not be conflated with a modern wind turbine, which is sometimes called a windmill but converts wind energy into electrical power. Windmills generally include a small occupiable structure holding the blades.
The rotating blades drive the local mechanical application. The defining output is mechanical work rather than electricity for distribution.
A small occupiable enclosure commonly houses the mechanism. Its presence is usual, while the wind-driven mechanical function establishes the distinction from a wind turbine.