Traditionally, "commodore" is the title for any officer assigned to command more than one ship at a time, even temporarily, much as "captain" is the traditional title for the commanding officer of a single ship even if the officer's official title in the service is a lower rank. As an official rank, a commodore typically commands a flotilla or squadron of ships as part of a larger task force or naval fleet commanded by an admiral. A commodore's ship is typically designated by the flying of a broad pennant, as opposed to an admiral's flag.
It is sometimes abbreviated: as "Cdre" in British Royal Navy, "CDRE" in the US Navy, "Cmdre" in the Royal Canadian Navy, "COMO" in the Spanish Navy and in some navies speaking the Spanish language, or "CMDE" as used in the Indian Navy or in some other Navies.
The rank of commodore derives from the Frenchcommandeur, which was one of the highest ranks in the orders of knighthood, and in military orders the title of the knight in charge of a commande (a local part of the order's territorial possessions).