A flag officer is a commissioned officer in a nation's armed forces senior enough to be entitled to fly a flag to mark where the officer exercises command. The term usually, but not exclusively, refers to the senior officers in an English-speaking nation's navy, specifically those who hold any of the admiral ranks; in some cases it applies also to those holding the rank of commodore. In U.S. usage it is additionally applied to Coast Guard officers and general officers in the U.S. Army, U.S. Air Force, and U.S. Marine Corps entitled to fly their own flags.
General usage 
The generic title of flag officer is used in several modern navies and associated units to denote those who hold the rank of rear admiral (or its equivalent) and above, also called "flag ranks"; in some navies, this also includes the rank of commodore. Flag officer corresponds to the generic terms general officer (used by land and some air forces to describe all grades of generals) and air officer (used by other air forces to describe all grades of air marshals and air commodores).
In the Canadian Forces, a flag officer (French: officier général, "general officer") is an admiral, vice-admiral, rear-admiral, or commodore, the naval equivalent of a general officer of the army or air force. It is a somewhat counterintuitive usage of the term, as only flag officers in command of commands or formations actually have their own flags (technically a commodore has only a broad pennant, not a flag), and army and air force generals in command of commands or formations also have their own flags, but are not called flag officers. Base commanders, usually full colonels, also have a pennant that flies from the mast or flagpole on the base, when resident, or on vehicles that carry them.
A flag officer's rank is denoted by a wide strip of gold braid on the cuff of the service dress tunic; one to four gold maple leaves over a crossed sword and baton, all beneath a royal crown, on epaulettes and shoulder boards; and two rows of gold oak leaves on the peak of the service cap. Since the Unification of the Canadian Forces in 1968, a flag officer's dress tunic had a single broad stripe on the sleeve and epaulettes. On May 5, 2010, however, the naval uniform dark dress tunic was adjusted—exterior epaulettes were removed, reverting to the sleeve ring and executive curl-rank insignia used by most navies; commodores' uniforms display a broad stripe, and each succeeding rank receives an additional sleeve ring. There are no epaulettes on the exterior of the tunic, but they are still worn on the uniform shirt underneath.
United Kingdom 
In the Royal Navy, there is a distinction between "flag officer" and "officer of flag rank". Formerly all officers promoted to flag rank were considered to be "flag officers" and the term is still widely used to refer to any officer of flag rank. Present usage is that all rear-admirals and above are officers of flag rank, but only those officers of flag rank who are authorised to fly a flag are formally called "flag officers", and have different flags for different ranks of admiral. Of the 39 officers of flag rank in the Royal Navy in 2006, very few were "flag officers" with entitlement to fly a flag. For example, Commander-in-Chief Fleet flies an admiral's flag whether ashore or afloat and is a "flag officer"; the chief of staff (support), a rear admiral, is not entitled to fly a flag and is an "officer of flag rank" rather than a "flag officer". List of fleets and major commands of the Royal Navy lists most admirals who were "flag officers". A flag officer's junior officer is often known as "Flags".
Equivalent ranks in the British Army and Royal Marines are called general officer rather than flag officers, and those in the Royal Air Force (as well as the rank of air commodore) are called air officers, although all are entitled to fly flags of rank.
United States 
In 1857 Congress created the title of "flag officer" as an actual rank of the Navy. The rank of flag officer was bestowed on senior navy captains who were assigned to lead a squadron of vessels in addition to command of their own ship. During the American Civil War, the Confederate States Navy also used the term. The 19th-century rank of "flag officer" was considered strictly temporary and became obsolete upon the creation and widespread usage of the equivalent naval rank of commodore; however, the term is still in use today to denote a category of naval officers equivalent to general officers.In 1862 Congress authorized American use of the title "admiral".
In the U.S. Army, Air Force, and Marine Corps, the term "flag officer" generally is applied to all general officers authorized to fly their own command flags—i.e., Brigadier General, or rank O-7, and above. However, as a matter of law, Title 10 of the United States Code makes a distinction between general officers and flag officers. Non-naval officers usually fly their flags from their headquarters, vessels, or vehicles, typically only for the most senior officer present. In the United States all flag and general officers must be nominated by the President and confirmed by the Senate; each subsequent promotion requires renomination and re-approval.
- Canada – National Defence: A-AD-200-000/AG-000 The Heritage Structure of the Canadian Forces, Chapter 14, Section 3.
- Canada - National Defence: "Navy Rank and Appointment Insignia: Navy"
- Note: The referenced website, above, has not yet been updated to reflect the change as of July 9, 2010.
- See e.g.King's Regulations and Admiralty Instructions Volume I 1913., §192
- .Flag Officers of the US Navy
- §101 of Title 10, US Code on law.cornell.edu
- Military and Associated Words. US Department of Defense, 2003. Answers.com 11 Jun. 2008.
- Army Regulation 840-10, Flags, Guidons, Streamers, Tabards, and Automobile and Aircraft Plates
- Department of the Army Institute of Heraldry website on General Officer Flags