The Collaborative International Dictionary of English v.0.48:

Post-captain \Post"-cap`tain\, n.
   A captain of a war vessel whose name appeared, or was
   "posted," in the seniority list of the British navy, as
   distinguished from a commander whose name was not so posted.
   The term was also used in the United States navy; but no such
   commission as post-captain was ever recognized in either
   service, and the term has fallen into disuse.
   [1913 Webster]