a watertight structure, usually of sheet piling, that encloses an area under water, pumped dry to enable construction work to be carried out. Below a certain depth a caisson is required
2.
(on a ship) a compartment separating two bulkheads or floors, as for insulation or to serve as a barrier against the escape of gas or oil
Often shortened to: coffer
cofferdam in American English
(ˈkɔfərˌdæm; ˈkɑfərˌdæm)
noun
1.
a watertight temporary structure in a river, lake, etc., for keeping the water from an enclosed area thathas been pumped dry so that a bridge foundation, dam, or pier may be constructed
2.
a watertight box or chamber attached to the side of a ship so that repairs can be made below the waterline
3.
an empty space serving as a protective barrier, as between two compartments of a ship