pontoon

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A floating structure, such as a flatbottom boat, that is used to support a bridge.
2. n. A floating structure serving as a dock.
3. n. A float on a seaplane.

Century Dictionary and Cyclopedia

1. n. In milit engin., a flat-bottomed boat, or any light framework or floating structure, used in the construction of a temporary bridge over a river. One form of pontoon is a hollow cylinder of tin-plate, with hemispherical ends, divided by several longitudinal and transverse partitions to act as braces and to prevent sinking if pierced by a shot or accidentally. Another is in the form of a decked canoe, consisting of a timber frame covered with sheetcopper, and formed in two distinct parts, which are locked together for use and dislocated for transportation, and also divided into air-tight chambers.
2. n. Nautical, a lighter; a low flat vessel resembling a barge, furnished with cranes, capstans, and other machinery, used in careening ships, chiefly in the Mediterranean.
3. n. In hydraulic engineering: A water-tight structure or frame placed beneath a submerged vessel and then filled with air to assist in refloating the vessel.
4. n. A water-tight structure which is sunk by filling with water and raised by pumping it out, used to close a sluiceway or entrance to a dock. Also spelled ponton.
5. n. In anatomy, a loop or knuckle of the small intestine: so called from the way it appears to float in the abdominal cavity. See the quotation under mesentery.
6. n. In brewing, one of the cleansing-rounds or cleansing-squares used for clarifying ale.
7. n. Same as catamaran, 4.

Wiktionary

1. n. card games A card game in which the object is to obtain cards whose value adds up to, or nearly to, 21 but not exceed it.
2. n. military A flat-bottomed boat used as a support for a temporary bridge.
3. n. A floating structure supporting a bridge or dock.
4. n. A box used to raise a sunken vessel.
5. n. A float of a seaplane.

GNU Webster's 1913

1. n. (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. n. (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.

WordNet 3.0

1. n. a float supporting a seaplane
2. n. (nautical) a floating structure (as a flat-bottomed boat) that serves as a dock or to support a bridge