keel

(nautical) A large beam along the underside of a ship’s hull from bow to stern.

(nautical) A rigid, flat piece of material anchored to the lowest part of the hull of a ship to give it greater control and stability.

(aeronautics) In a dirigible, a construction similar in form and use to a ship's keel; in an aeroplane, a fin or fixed surface employed to increase stability and to hold the machine to its course.

(by extension) The rigid bottom part of something else, especially an iceberg.

(nautical) A type of flat-bottomed boat.

(zoology) The periphery of a whorl extended to form a more or less flattened plate; a prominent spiral ridge.

(botany) The two lowest petals of the corolla of a papilionaceous flower, united and enclosing the stamens and pistil; a carina.