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;
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.
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.
WordNet (r) 3.0 (2006):
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