Post-captain
Post-captain is an obsolete alternative form of the rank of captain in the Royal Navy.
The term served to distinguish those who were captains by rank from:
Officers in command of a naval vessel, who were (and still are) addressed as captain regardless of rank;
Commanders, who received the title of captain as a courtesy, whether they currently had a command or not (e.g. the fictional Captain Jack Aubrey in Master and Commander or the fictional Captain Horatio Hornblower in Hornblower and the Hotspur); this custom is now defunct.
Once an officer had been promoted to post-captain, his further promotion was strictly by seniority; if he could avoid death or disgrace, he would eventually become an admiral (even if only a yellow admiral).
In the Royal Navy of the time, an officer might have a rank, but not a command. Until the officer had a command, he was "on the beach" and on half-pay. An officer who was promoted from commander was a captain, but until he was given a command, he was on half-pay. Once the captain was given a command, his name was "posted" in the London Gazette.