The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cofferdam
    n 1: large watertight chamber used for construction under water
         [syn: caisson, pneumatic caisson, cofferdam]