The Collaborative International Dictionary of English v.0.48:

Centerboard \Cen"ter*board`\, Centreboard \Cen"tre*board\,
   (s[e^]n"t[~e]r*b[=o]rd`), n. (Naut.)
   A retractable or sliding keel used on sailboats, 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 drifting to leeward when the vessel is beating to
   windward. It is used in vessels of all sizes along the coast
   of the United States

   Syn: centreboard, drop keel, sliding keel.
        [1913 Webster]

WordNet (r) 3.0 (2006):

centerboard
    n 1: a retractable fin keel used on sailboats to prevent
         drifting to leeward [syn: centerboard, centreboard,
         drop keel, sliding keel]