a strongly fortified seaport town of France, department of Le Nord, and arrondissement of Dunkerque, on the Aa, near its mouth, in the English Channel. It is chiefly known for its fortifications, which are of great strength, having been constructed by Vauban; and, being in a low marshy situation, protected from the sea by dunes or sandhills, the surrounding country may be laid under water at pleasure. The inhabitants are chiefly engaged in the cod, mackerel, and herring fisheries. Pop. 5582.