sternpost

The principal upright post at the stern of a vessel, usually serving to support the rudder.

The principal piece of timber or iron in a vessel’s stern-frame.

A straight piece of timber, or an iron bar or beam, erected on the extremity of the keel to support the rudder, and receive the ends of the planks or plates of the vessel.

A timber or steel bar extending from the keel to the main deck at the stern of a vessel.

(nautical) the principal upright timber at the stern of a vessel