sponsonPronunciation: /ˈspɒns(ə)n/Definition of sponson noun a projection on the side of a boat, ship, or seaplane, in particular: a gun platform standing out from a warship’s side. a short subsidiary wing that serves to stabilize a seaplane. a buoyancy chamber fitted to a boat’s hull, especially on a canoe. a triangular platform supporting the wheel on a paddle steamer. Origin: mid 19th century: of unknown originsponson in other Oxford dictionaries Definition of sponson in the US English dictionary