A keel is a structural element on a watercraft that runs longitudinally along the centerline of the ship, from the bow to the stern. It is the bottom-most structural member around which the hull of a ship is built. The keel is often the first part of a ships hull to be constructed, and laying the keel, or placing the keel in the cradle in which the ship will be built, may mark the start time of its construction. The keel has several functions, including:

• Hydrodynamic and counterbalancing purpose: On some sailboats, the keel may have a hydrodynamic and counterbalancing purpose.
• Stability and control: The keel helps give the boat greater stability and control while moving forward.
• Preventing sideways movement: The keel prevents the boat from being blown sideways by the wind.
• Holding ballast: The keel holds the ballast that keeps the boat right-side up.

There are different types of keels, including:

• Structural keels: These are the bottom-most longitudinal structural elements on a watercraft.
• Full keels: These are vertical downward extensions of the boat’s hull, narrowly V-shaped, and usually ballasted or weighted for stability and lateral resistance.
• Fin keels: These are narrow plates fixed midships to the keel of a shallow boat and projecting downward to provide lateral resistance. They are intended both to steady the boat and to make it handy to steer.
• Bilge keels: These are one of a pair of longitudinal plates that project from the sides of a ship or boat and run parallel to the center keel. They are intended to provide lateral resistance.

The word "keel" comes from Old English cēol, Old Norse kjóll, which means "ship" or "keel".