Definition from Wiktionary, the free dictionary
Etymology 1 
pontoon (plural pontoons)
- (military) A flat-bottomed boat used as a support for a temporary bridge.
- A floating structure supporting a bridge or dock.
- A box used to raise a sunken vessel.
- A float of a seaplane.
- (floating structure used as a bridge): pontoon bridge
support for a temporary bridge
floating structure supporting a bridge or dock
box used to raise a sunken vessel
Etymology 2 
- (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.
- (card game): vingt-et-un