pontoon

n. [ F. ponton (cf. It. pontone), from L. ponto, -onis, fr. pons, pontis, a bridge, perhaps originally, a way, path: cf. Gr. &unr_; path, Skr. path, pathi, panthan. Cf. Punt a boat. ] 1. (Mil.) A wooden flat-bottomed boat, a metallic cylinder, or a frame covered with canvas, India rubber, etc., forming a portable float, used in building bridges quickly for the passage of troops. [ 1913 Webster ]

2. (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. [ 1913 Webster ]

Pontoon bridge, a bridge formed with pontoons. --
Pontoon train, the carriages of the pontoons, and the materials they carry for making a pontoon bridge. [ 1913 Webster ]

☞ The French spelling ponton often appears in scientific works, but pontoon is more common form. [ 1913 Webster ]