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. They are thus the equivalent of an executive officer in the United States Army. This terminology is also used in many other Commonwealth armies and other nations.
The second-in-command of a battalion or regiment is usually a major. The second-in-command of a company, squadron, or artillery battery (in which they are 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.
In common usage, the term refers to a deputy. Its basic meaning is "someone who relieves a commander".