Lieutenant Commander

Lieutenant commander was a commissioned officer rank, the equivalent of which was used by the service organizations of many civilizations. It was common in informal circumstances to refer to lieutenant commanders simply as "commander" by both those under their command and their commanding officers.
A lieutenant commander was a grade between the lieutenant and commander grades with the rank equivalent to a major in the marine or ground forces. Lieutenant commanders normally served as senior department heads, executive officers on smaller vessels, and in some cases as starship commanders. Another position that could be held by a lieutenant commander was as an instructor for Starfleet's Advanced Tactical Training.