The Second-in-Command (2i/c) is the deputy commander of any British Army unit, from battalion or regiment downwards. He is thus the equivalent of an Executive Officer in the United States Army. The 2i/c of a battalion or regiment is usually a major, the 2i/c of a company, squadron or artillery battery (in which he is called the Battery Captain) is usually a captain (although infantry company 2i/cs were usually lieutenants until after the Second World War), the 2i/c of a platoon or troop is the platoon or troop sergeant, and the 2i/c of a section is usually a lance-corporal.

Less formally, second-in-command is a term used, usually unofficially and sometimes jocularly, for any deputy to a chief officer or head of an organisation.