In the units of many military forces, the Executive Officer (XO) is the second-in-command, reporting to the commanding officer (CO).
United Kingdom
The term is not used in the British Army or Royal Marines (RM), in which the designation Second-in-Command (2i/c) is used as a formal appointment. In the Royal Air Force (RAF), the term is informally used between officers and airmen, referring to the officer who is second-in-command. It is formally used in the Royal Navy (RN), however. In smaller vessels, such as submarines and frigates, the Executive Officer also holds the position of First Lieutenant. Originally, the Second-in-Command was usually referred to as First Lieutenant (or as "Number One"), although it is becoming more common to hear the term XO. On larger ships, in which the XO holds the rank of Commander, the XO is usually referred to simply as "the Commander". The XO heads the executive department.