Noun
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.
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.
Source: Webster's dictionaryWe can't cross that bridge until we come to it, but I always like to lay down a pontoon ahead of time. Bernard Baruch
After a pontoon bridge they constructed was swamped and swept away by the flood waters, Groves was accused of negligence. Source: Internet
As Maxentius had probably partially destroyed the bridge during his preparations for a siege, he had a wooden or pontoon bridge constructed to get his army across the river. Source: Internet
Bureaucratic delays prevented Burnside from receiving the necessary pontoon bridges in time and Lee moved his army to block the crossings. Source: Internet
Burnside first requisitioned the pontoon bridging (along with many other provisions) on November 7 when he detailed his plan to Halleck. Source: Internet
Cable ferries (next section) are usually pontoon ferries, but pontoon ferries on larger rivers are motorised and able to be steered independently like a boat. Source: Internet