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]

The Collaborative International Dictionary of English v.0.48:

Bridge \Bridge\ (br[i^]j), n. [OE. brig, brigge, brug, brugge,
   AS. brycg, bricg; akin to Fries. bregge, D. brug, OHG.
   brucca, G. br["u]cke, Icel. bryggja pier, bridge, Sw. brygga,
   Dan. brygge, and prob. Icel. br[=u] bridge, Sw. & Dan. bro
   bridge, pavement, and possibly to E. brow.]
   1. A structure, usually of wood, stone, brick, or iron,
      erected over a river or other water course, or over a
      chasm, railroad, etc., to make a passageway from one bank
      to the other.
      [1913 Webster]

   2. Anything supported at the ends, which serves to keep some
      other thing from resting upon the object spanned, as in
      engraving, watchmaking, etc., or which forms a platform or
      staging over which something passes or is conveyed.
      [1913 Webster]

   3. (Mus.) The small arch or bar at right angles to the
      strings of a violin, guitar, etc., serving of raise them
      and transmit their vibrations to the body of the
      instrument.
      [1913 Webster]

   4. (Elec.) A device to measure the resistance of a wire or
      other conductor forming part of an electric circuit.
      [1913 Webster]

   5. A low wall or vertical partition in the fire chamber of a
      furnace, for deflecting flame, etc.; -- usually called a
      bridge wall.
      [1913 Webster]

   Aqueduct bridge. See Aqueduct.

   Asses' bridge, Bascule bridge, Bateau bridge. See under
      Ass, Bascule, Bateau.

   Bridge of a steamer (Naut.), a narrow platform across the
      deck, above the rail, for the convenience of the officer
      in charge of the ship; in paddlewheel vessels it connects
      the paddle boxes.

   Bridge of the nose, the upper, bony part of the nose.

   Cantalever bridge. See under Cantalever.

   Draw bridge. See Drawbridge.

   Flying bridge, a temporary bridge suspended or floating, as
      for the passage of armies; also, a floating structure
      connected by a cable with an anchor or pier up stream, and
      made to pass from bank to bank by the action of the
      current or other means.

   Girder bridge or Truss bridge, a bridge formed by
      girders, or by trusses resting upon abutments or piers.

   Lattice bridge, a bridge formed by lattice girders.

   Pontoon bridge, Ponton bridge. See under Pontoon.

   Skew bridge, a bridge built obliquely from bank to bank, as
      sometimes required in railway engineering.

   Suspension bridge. See under Suspension.

   Trestle bridge, a bridge formed of a series of short,
      simple girders resting on trestles.

   Tubular bridge, a bridge in the form of a hollow trunk or
      rectangular tube, with cellular walls made of iron plates
      riveted together, as the Britannia bridge over the Menai
      Strait, and the Victoria bridge at Montreal.

   Wheatstone's bridge (Elec.), a device for the measurement
      of resistances, so called because the balance between the
      resistances to be measured is indicated by the absence of
      a current in a certain wire forming a bridge or connection
      between two points of the apparatus; -- invented by Sir
      Charles Wheatstone.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pontoon bridge
    n 1: a temporary bridge built over a series of pontoons [syn:
         pontoon bridge, bateau bridge, floating bridge]