n.1. Nautical a. The principal structural member of a boat or ship, running along the center of the hull from bow to stern, to which the ribs are attached.
b. A projecting ridge or fin on the bottom of the hull of a boat or ship that improves directional control and is often weighted for added stability.
2. The principal structural member of an aircraft, resembling a ship's keel in shape and function.
3. A structure, such as the breastbone of a bird, that resembles a ship's keel in function or shape.
4. A pair of united petals in certain flowers, as those of many members of the pea family.