an aquarium filter with the media enclosed in a plastic can outside the tank, with water leaving and entering the can through hoses leading to and from the aquarium.
These filters differ from the "hang on the back of your tank" filters in that they are external to the tank. They have an input and output hose. Water leaves the tank, enters the canister for filtration, then re-enters the tank. These types of filters are generally more efficient than "hang on" filters and are desirable for aesthetics as the filter can be hidden.
An external filter, which pushes water through a canister containing various filter media.