Commander

Commander is a military rank, the equivalent of which is used by the service organizations of many civilizations.

Use in Starfleet

The Starfleet rank of Commander sits between the ranks of Lieutenant Commander and Captain and is the second most senior line officer rank. The rank of Commander is equivalent to the rank of the Starfleet Marine Corp rank of Lieutenant Colonel.

In Starfleet the rank of Commander is typically held by the First officer on Starships and Bases. Commanders can also been assigned to command smaller Starships, Outposts, Space Stations and Starbases. The rank of Commander can also be awarded to officers who hold a senior position on the starship that they serve on, provided that they meet all of the necessary requirements that includes taking and passing the Bridge Officer's Test.

Proper naval parlance says that, even if a naval vessel is commanded by an officer whose rank is commander, that officer is still able to be referred to as a captain while on the deck of their own vessel.

Advancement

As Commander's prove their leadership ability over time, they are considered for the difficult and highly competitive promotion to the rank of Captain.