Captain

Captain was a commissioned officer rank, the equivalent of which was used by the service organizations of many civilizations. The title of captain was often used by vessel commanders and, as a naval rank, in many Earth navies and Starfleet, was above commander.
Captains of starships, often located hundreds of light years away from and out of communications with higher authority, needed to be able to function autonomously and make independent command decisions affecting Federation policy and countless lives. Accordingly, few Starfleet officers ever gained the captaincy of a starship, the result of decades of sustained excellence as a Starfleet leader.