Source : Webster's Revised Unabridged Dictionary (1913)

Centerboard \Cen"ter*board`\, Centreboard \Cen"tre*board\,, n.
   (Naut.)
   A movable or sliding keel formed of a broad board or slab of
   wood or metal which may be raised into a water-tight case
   amidships, when in shallow water, or may be lowered to
   increase the area of lateral resistance and prevent leeway
   when the vessel is beating to windward. It is used in vessels
   of all sizes along the coast of the United States

Source : WordNet®

centreboard
     n : a retractable fin keel used on sailboats to prevent drifting
         to leeward [syn: {centerboard}, {drop keel}, {sliding
         keel}]