The Collaborative International Dictionary of English v.0.48:

Sternpost \Stern"post`\, n. (Naut.)
   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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sternpost
    n 1: (nautical) the principal upright timber at the stern of a
         vessel