It is sometimes abbreviated as Cdre, CDRE or COMO.
The rank of Commodore derives from the French commandeur, which was one of the highest ranks in orders of knighthood, and in military orders the title of the knight in charge of a commenda (a local part of the order's territorial possessions).
The Royal Netherlands Navy also used the rank of commandeur from the end of the 16th century for a variety of temporary positions, until it became a conventional permanent rank in 1955. The Royal Netherlands Air Force has adopted the English spelling of Commodore for an equivalent rank.
The rank of Commodore was at first a position created as a temporary title to be bestowed upon Captains who commanded squadrons of more than one vessel. In many navies, the rank of Commodore was merely viewed as a Senior Captain position, whereas other naval services bestowed upon the rank of Commodore the prestige of flag officer status - Commodore is the highest rank in the Irish Naval Service, for example, and is held by only one person. In the Royal Navy, the position was introduced to combat the cost of appointing more Admirals - a costly business with a fleet as large as the Royal Navy's at that time.
In 1899 the substantive rank of Commodore was discontinued in the United States Navy and United States Coast Guard, but revived during World War II. It was discontinued as a rank in these services during the postwar period, but as an appointment, the title "Commodore" was then used to identify senior U.S. Navy Captains who commanded squadrons of more than one vessel or functional air wings or air groups that were not part of a carrier air wing or air group. Concurrently, until the early 1980s, U.S. Navy and U.S. Coast Guard Captains selected for promotion to the rank of Rear Admiral (Lower Half), would wear the same insignia as Rear Admiral (Upper Half), i.e., two stars or sleeve braid of one wide and one narrow gold stripe, even though they were actually only equivalent to one star officers. To correct this inequity, the rank of Commodore as a single star Flag Officer was reinstated by both services in the early 1980s. This immediately caused confusion with those senior U.S. Navy Captains commanding destroyer squadrons, submarine squadrons, functional air wings and air groups, etc., who held the temporary "title" of Commodore. As a result of this confusion, the services soon renamed the new one star rank as Commodore Admiral (CADM) within the first six months following the rank's reintroduction. This was considered an awkward title and the rank was renamed a few months later to its current title of Rear Admiral (Lower Half), or RDML. The "title" of Commodore continues to be used in the U.S. Navy for those senior Captains in command of organizations consisting of groups of ships or submarines organized into squadrons, air wings or air groups of aviation squadrons other than carrier air wings, special warfare (SEAL) groups, and construction battalion (SeaBee) regiments. Although not Flag Officers, modern day Commodores in the U.S. Navy will rate a blue and white command pennant that is normally flown at their headquarters facilities ashore or from ships they may be embarked aboard.
Many air forces, use the rank of Air Commodore. This rank was first used by the Royal Air Force and is now used in many countries such as India, Australia, New Zealand, Greece, Thailand, Zimbabwe, Pakistan and Nigeria. It is the equivalent rank to the navy rank of "commodore", and the army ranks of brigadier and brigadier general.
The German air force used the concept of a unit Commodore, although this was a unit command appointment rather than a rank.
In the United States Coast Guard Auxiliary, the senior elected officer of the organization is the National Commodore, while there are Commodores elected for the Atlantic and Pacific regions.
In the U.S. Sea Scouting program (which is part of the Boy Scouts of America), all National, Regional, Area, and Council committee chairs are titled as Commodore, while senior committee members are addressed as Vice Commodore. Ship Committee chairs do not hold this recognition.