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Second-in-command (2i/c or 2IC) is a title denoting that the holder of the title is the second-highest authority within a certain organisation.

In the British Army or Royal Marines, the second-in-command is the deputy commander of a unit, from battalion or regiment downwards. This terminology is also used in many other Commonwealth armies and other nations. The equivalent appointment in the United States Army is the executive officer.^[1]

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 the Royal Navy and Commonwealth navies, the second-in-command of a vessel, regardless of rank, is known as the first lieutenant or executive officer.

In common usage, the term refers to a deputy. Its basic meaning is "someone who relieves a commander".