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While executive officer literally refers to a person responsible for the performance of duties involved in running an organization, the exact meaning of the role is variable, depending on the organization.

In the units of many military forces, the Executive Officer (XO) is the second-in-command, reporting to the Commanding Officer (CO). In the United States Army and Marine Corps, for example, there are XO billets in each company, battalion, and brigade, though not at higher levels of command. The XO billet is not a command; rather it is considered staff. The XO is typically responsible for the management of day-to-day activities, such as maintenance and logistics, freeing the unit commander to concentrate on tactical planning and execution. The XO also takes charge in the absence of the CO. While the experience gained as an XO is highly beneficial for an officer's professional development, never serving in the position will not preclude an officer from commanding later.

The United States Air Force (USAF) uses the titles of "Vice Commander" or "Deputy Commander" for an officer who is second-in-command (or Director of Operations on the squadron level), while "Executive Officer" is used for an officer who handles most of the administrative workload for a commander with a rank of Colonel or above.

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, nor in the Royal Air Force (RAF). It is 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 and was originally usually referred to as such (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.