Lieutenant commander

Lieutenant Commander (Lieutenant-Commander in some rank schemes) is a military rank, the equivalent of which is used by the service organizations of many civilizations. A traditional naval rank, Lieutenant Commander is the grade between lieutenant and commander grades. The correct form of address for this rank is "Commander"; the term "Lieutenant Commander" would only be used ceremonially or in printed orders or communications.

Unlike other ranks such as Lieutenant Colonel or Lieutenant General, a Lieutenant Commander is not a lesser version of Commander, but rather is a more senior grade of Lieutenant (other navies, such as those used in Germany and Russia, use the term 'Captain Lieutenant'). The rank was originally created because some Lieutenants were being given command of smaller vessels in the United States Navy (where these officers were called 'Lieutenant Commanding' or 'Lieutenant Commandant') and the British Royal Navy (there referred to as 'Lieutenant in Command' or 'Lieutenant and Commander'); the US Navy adopted the resulting new rank of Lieutenant Commander in 1862, and the Royal Navy followed suit in 1914.

The rank is equivalent to the military rank of Major, the Cardassian rank of Glinn, and the Romulan rank of Centurion.

The Federation Starfleet uses this rank, although its 'predecessor', the Earth Starfleet may not have had a rank between Lieutenant and Commander. This rank also existed in the systems used by many older Earth agencies, such as the United States armed forces.

From Memory Alpha -- the free Star Trek reference