Source : Webster's Revised Unabridged Dictionary (1913)

Pontoon \Pon*toon"\, n. [F. ponton (cf. It. pontone), from L.
   ponto, -onis, fr. pons, pontis, a bridge, perhaps originally,
   a way, path: cf. Gr. ? 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.

   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.

   {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.

Source : WordNet®

pontoon
     n 1: (nautical) a floating structure (as a flat-bottomed boat)
          that serves as a dock or to support a bridge
     2: a float supporting a seaplane