Impeller: the rotating part of a centrifugal pump or mixer, compressor, or other machine designed to move a fluid by rotation.

Propeller: a mechanical device for propelling a boat or aircraft, consisting of a revolving shaft with two or more broad, angled blades attached to it.

At Dynamix Agitators, we use impellers for most mixing applications. A propeller is used for high shear, high RPM applications. Ex: Mixing powder and water.