A fan is a powered machine that circulates air. A fan is a rotating arrangement of vanes or blades that act on the air and are typically made of wood, plastic, or metal. An impeller, rotor, or runner is a rotating assembly of blades and a hub. It is usually contained within some kind of housing or case.  This can be used to direct airflow or to increase safety by preventing objects from colliding with the fan blades. Most fans are powered by electric motors, but other sources of power, such as hydraulic motors, handcranks, and internal combustion engines, may be used.
Mechanically, a fan can be any revolving vane or vanes used to generate air currents. Fans produce high-volume, low-pressure air flows (though higher than ambient pressure), whereas compressors produce high-pressure air flows at a low volume. When exposed to an air-fluid stream, a fan blade will frequently rotate, and devices that take advantage of this, such as anemometers and wind turbines, frequently have fan-like designs.