Second-in-command 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.
The second-in-command of a battalion or regiment is usually a major. The second-in-command of a company, squadron, or artillery battery is usually a captain, 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.

Other meanings and uses

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