'Centerboard' definitions:

Definition of 'centerboard'

noun

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

From: GCIDE

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]