Broadly speaking a subsidiary structure extending from the side of the ship. Nowadays most commonly used to describe the inflatable hull sections of a Rigid Inflatable Boat (RIB). The port and starboard floats of a trimaran would also be called sponsons, and when used of small pleasure craft the word normally implies some sort of stabilising float. But in ships a sponson may be a gun platform jutting from the side, or the platforms forward and aft of the paddle-wheels on a steamer so equipped.