Learner's definition of PONTOON
: a large hollow container filled with air that is used to make a structure (such as a boat, airplane, or bridge) float on top of water
The boat had two pontoons.
They built a pontoon bridge. [=a bridge that floats on the water and is held up by pontoons]
a pontoon plane [=an airplane that has pontoons on the bottom so that it can float on water]