The Second-in-Command (2i/c or 2IC) is the second commander, the person that is in charge after the commander, of any British Army or British Royal Marines unit/group. He or she is the same as an Executive Officer in the United States Army. This term or word is also used in many other Commonwealth militaries.
The Second-in-Command of a group is usually a Major. The Second-in-Command of a smaller group is usually a Captain (although infantry groups' Second-in-Command's were usually Lieutenants until after the Second World War), the Second-in-Command of an even smaller group is the Sergeant of the group, and the Second-in-Command of a section is usually a Lance Corporal.
"Second-in-command" is a term/word used, usually as a joke, for any deputy or person that is right below the commander. Its basic meaning is "someone that relieves a commander".