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- For the film, see Second in Command.
The second-in-command (2i/c or 2IC) is the deputy commander of any British Army or Royal Marines unit, from battalion or regiment downwards. He or she is thus the equivalent of an executive officer in the United States Army. This terminology is also used in many other Commonwealth armies.
The second-in-command of a battalion is usually a major and the second-in-command of a regiment is a lieutenant colonel. The second-in-command of a company, squadron or artillery battery (in which he or she is called the battery captain) is usually a captain (although infantry company second-in-commands were usually lieutenants until after the Second World War), the second-in-command of a platoon or troop is the platoon or troop sergeant, and the second-in-command of a section is usually a lance corporal.
Other meanings/uses 
In common usage, the term refers to a deputy. Its basic meaning is "someone who relieves a commander". When the first-in-command is absent, the second-in-command will temporarily assume the duties thereof. Some have asserted that there exists the theoretical concept of a third-in-command whereby the so-called "third-in-command" assumes the duties of the absent second-in-command, excepting, of course, when the first-in-command is not absent.