pontoon

noun

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

   “Thanks to the previous experience of the chief performers, to careful preparation and rehearsal, and to perfect weather conditions, they watched in breath-bated silence the spectacle of the huge bulk moving without a hitch into position over the temporary pier caps on which it settled as water ballast was admitted to the pontoons.”

2. (broadly)A bridge with floating supports.
3. A box used to raise a sunken vessel.

   “Near-synonyms: caisson, camel, sea camel”

4. A float of a seaplane.
5. A lighter or barge used for loading or unloading ships.
6. (uncountable)A card game in which the object is to obtain cards whose value adds up to, or nearly to, 21 but not exceed it.