The Collaborative International Dictionary of English v.0.48:

Breakwater \Break"wa`ter\, n.
   Any structure or contrivance, as a mole, or a wall at the
   mouth of a harbor, to break the force of waves, and afford
   protection from their violence.
   [1913 Webster]

WordNet (r) 3.0 (2006):

breakwater
    n 1: a protective structure of stone or concrete; extends from
         shore into the water to prevent a beach from washing away
         [syn: breakwater, groin, groyne, mole, bulwark,
         seawall, jetty]