An executive officer is a person who is principally responsible for leading all or part of an organization, although the exact nature of the role varies depending on the organization.

In many militaries and police forces, an executive officer (XO) is the second-in-command, reporting to the commanding officer (CO). The XO is typically responsible for the management of day-to-day activities, freeing the commander to concentrate on strategy and planning the unit's next move.

While there is no clear line between principal executive officers and inferior executive officers, principal officers are high-level officials in the executive branch of U.S. government such as department heads of independent agencies. In Humphrey's Executor v. United States, 295 U.S. 602 (1935), the Court distinguished between executive officers and quasi-legislative or quasi-judicial officers by stating that the former serve at the pleasure of the president and may be removed at their discretion. The latter are removed only with procedures consistent with statutory conditions enacted by Congress. The decision by the Court was that the Federal Trade Commission was a quasi-legislative body because of other powers it had, and therefore, the president could not fire an FTC member for political reasons. Congress can't retain removal power over officials with executive function (Bowsher v. Synar). However, statutes can restrict removal if not purely executive (Humphrey's executor), but can't restrict removal of purely executive officer (Myers v. United States, 272 U.S. 52 (1926)). The standard is whether restriction "impedes the president's ability to perform his constitutional duty" (Morrison v. Olson, 487 U.S. 654 (1988)).

In business, executive officers are usually the top staff members of a corporation, the chief executive officer (CEO) being the best-known type. The definition varies; for instance, the California Corporate Disclosure Act defines "executive officers" as the five most highly compensated officers not also sitting on the board of directors. In many insurance policies, executive officer means, in the case of a corporation, any chairman, chief executive officer, chief financial officer, chief operating officer, president, or general counsel. In the case of a sole proprietorship, an executive officer is the sole proprietor. In the case of a partnership, an executive officer is a managing partner, senior partner, or administrative partner. In the case of a limited liability company, an executive officer is any member, manager, or officer.

In charities, voluntary sectors and Nonprofit organizations, the executive officers are those appointed to drive the day-to-day decisions of the organization. This is normally a formal appointment made by the executive board of trustees. Specific responsibilities and scope vary and are usually called out in a governance document.

In the units of some military forces, the executive officer is the second-in-command, reporting to the commanding officer (CO).

In most non-naval military services that are land-based (except the U.S. Army, where an executive officer is the second-in-command of certain units) or in joint military organizations, the executive officer is an administrative staff position versus a command position. XOs in these positions typically assist a commander or deputy commander (or in the case of joint staffs or joint commands, a director) by managing day-to-day activities such as management of the senior officer's schedule, screening of documents or other products, and oversight of the senior officer's administrative support staff.

Management of individual assets in the airline industry is derived from the military terminology, where an executive officer or first officer, is the second in command of the aircraft. In a fixed wing aircraft, the first officer sits in the right-hand seat, but on a rotary wing aircraft, they sit on the left. Management of the airline as a whole is more in-line with the corporate example above.