a foil that converts lateral slipping of the boat into a lateral force that resists the slipping

(Or centreplate, or drop keel). A movable plate of iron, wood, or lead that may be let down below the keel, about midships, in order to reduce lee-way when under sail. The centreboard is housed within a watertight compartment termed the trunk, or keel box.

A removable keel used to resist leeway.

A movable, fin-shaped protrusion under the hull that prevents a boat from sliding sideways and is used to right capsized dinghies.

A centreboard is a retractable keel which pivots out of a slot in the hull of a sailboat, known as a centerboard trunk. The pivoting ability allows the centreboard to be raised to operate in shallow waters, to reduce drag when the full area of the centreboard isn't needed, or to remove the boat from the water as on a trailer. A daggerboard is similar but slides vertically rather than pivoting.