A seawall is a strong wall built along the edge of the sea, so the water cannot wash onto the land or eat away the shore. You see them in towns by the sea, along harbours, and in places where storms push the water high. It is simply the ordinary words sea and wall stuck together; English has had this compound for a very long time (Old English already had sǣweall). Though actually, people sometimes use seawall and breakwater as if they were the same thing. A seawall usually stands on the land’s edge to keep the sea off. A breakwater more often stands out in the water to break the waves before they reach the shore.