Source : Webster's Revised Unabridged Dictionary (1913)

Cofferdam \Cof"fer*dam\, n.
   A water-tight inclosure, as of piles packed with clay, from
   which the water is pumped to expose the bottom (of a river,
   etc.) and permit the laying of foundations, building of
   piers, etc.

Source : WordNet®

cofferdam
     n : large watertight chamber used for construction under water
         [syn: {caisson}, {pneumatic caisson}]