German Navy
In the German Kriegsmarine of the Second World War, Grand Admiral (Großadmiral) was considered the highest Naval rank. The position was the equivalent of an Admiral of the Navy or a United States Fleet Admiral.
Karl Dönitz, who commanded the German U-Boat Fleet, held the rank of Grand Admiral. The rank was also held by Erich Raeder.
Star Wars
The rank of Grand Admiral also has appeared in science fiction literature, most significiant of which is the fanon literature of Star Wars. Grand Admiral Thrawn, of the Timothy Zahn novels, is one such science fiction character who held this highest of naval ranks.
In Star Wars, Grand Admiral is a supreme military rank, outranking all other political and military personnel, including Imperial Moffs.
Grand Admirals are directly appointed by Emperor Palpatine and answer directly to the Emperor, or his authorized executors, in all matters. In the Imperial Army, the equivalent of a Grand Admiral is known as Grand General.
At the time of the Battle of Endor, marked by many as the beginning of the fall of the Galactic Empire, there were a total of twelve Grand Admirals and Grand Generals in service.
The insignia for Grand Admiral is seven blue and red squares, with two code cylinders, worn on a white uniform.