What does "pontoon" mean?

Pontoon: (Naut.) A low, flat vessel, resembling a barge, furnished with cranes, capstans, and other machinery, used in careening ships, raising weights, drawing piles, etc., chiefly in the Mediterranean; a lighter. Pontoon bridge, a bridge formed with pontoons. -- Pontoon train, the carriages of the pontoons, and the materials they carry for making a pontoon bridge. Note: The French spelling ponton often appears in scientific works, but pontoon is more common form.