Counter Admiral is an Anglicisation of a naval rank found in most navies of the world. This Anglicisation is, however, rarely used, as the rank is usually kept in the original language or translated to Rear Admiral or in the United States, Commodore.
Traditionally the commander of a fleet, the Admiral usually stationed his flagship in the middle of the formation, with his primary deputy, the Vice Admiral, in the vanguard and his second deputy, the Counter Admiral, in the rear, leading the reserve. The reason for the designation was that in the event the formation had to reverse (or "counter") direction, the Counter Admiral would lead it.
In most modern navies, Counter Admiral is generally, although not always, the lowest flag officer rank (in the German navy, for instance, Flotillenadmiral ranks below Konteradmiral).
