A fan is a machine used to create flow within a fluid, typically a gas such as air. The fan consists of a rotating arrangement of vanes or blades which act on the air.

The rotating assembly of blades and hub is known as an impeller, a rotor, or a runner. Usually, it is contained within some form of housing or case. 

This may direct the airflow or increase safety by preventing objects from contacting the fan blades.  Fans produce air flows with high volume and low pressure (although higher than ambient pressure).