Commander

Commander is a commissioned rank in many militaries. 
Commanders in the United Federation Starfleet serve as executive officers or department heads in the senior staff of large commands or as the commanding officers of medium-sized commands. They have been trained in a post-graduate command-track environment that allows them to take command of a Starfleet vessel, should the need arise. Commander is also the lowest rank permitted of a Starfleet starbase commanding officer where the base is a major hub of strategic importance.