Definition of "pontoon"

A flat-bottomed boat or other floating structure used as a buoyant support for a temporary bridge, dock or landing stage.  examples

(by extension) A bridge with floating supports.  examples

A box used to raise a sunken vessel.  examples

A float of a seaplane.  examples

A lighter or barge used for loading or unloading ships.  examples

(card games) A card game in which the object is to obtain cards whose value adds up to, or nearly to, 21 but not exceed it.  examples