The Collaborative International Dictionary of English v.0.48:

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.
      [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]

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

WordNet (r) 3.0 (2006):

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