Lieutenant Commander

Lieutenant Commander was a military rank found in a navy, junior to a Commander. Its army equivalent was Major.

Galactic Empire
In the Imperial Navy, a Lieutenant Commander was a command-level rank located in between Lieutenant and Commander. They had demonstrated further tactical thinking and command ability than Lieutenants, and were given certain levels of authority in the Navy.

On a ship the size of a, a Lieutenant Commander commanded a certain section of the ship or act as an assistant to a Commander or Captain. On smaller scale vessels, a Lieutenant Commander could act as Executive Officer, or even Commanding Officer, if the situation called for it.

New Republic
The rank was also used in the New Republic Starfleet.