Fleet captain

Fleet captain is a rare military title that may be bestowed upon senior naval captains for a variety of reasons, the most common of which is to temporarily take command of more than one vessel. The rank can, therefore, be interpreted as a close equivalent to commodore or to the 19th century rank of flag officer. The main distinction of a fleet captain is that normally the title is strictly temporary for the duration of a single mission.

Historical usage
Fleet captain has appeared in the navies of Germany (Flottenkapitän) and the United Kingdom as an unofficial title dating back to the 18th century. In the Royal Navy, during the 18th and 19th centuries, an admiral's flagship might also have a "captain of the fleet", who would be ranked between the admiral and the ship's captain (or "flag captain"). The "captain of the fleet" would be listed as the ship's "first captain" and would serve as the admiral's chief-of staff. The "flag captain" would be listed as the ship's "second captain" and retain actual command of the vessel.

Science fiction
Fleet captain has appeared in several science fiction sources, the most notable of which is Star Trek. As a Starfleet officer rank, there have only been two fleet captains shown in the entirety of Star Trek, both from Star Trek: The Original Series: Garth of Izar and Christopher Pike.