Counter Admiral is an Anglification of a naval rank found in some European navies; e.g. in the Deutsche Marine: Konteradmiral. In meaning it is the equivalent of the British and American rank of Rear Admiral, though it is no longer always the equivalent pay grade.
Traditionally, the commander of a fleet, the Admiral, usually stationed his flagship in the center of formation, with his primary deputy, the Vice Admiral, in the vanguard and his second deputy, the Counter Admiral, in the rear, leading the reserve.
Admiral is the rank, or part of the name of the ranks, of the highest naval officers.
The word Admiral has today come to be almost exclusively associated with the highest naval rank in most of the world's navies, equivalent to the Army rank of (Full) General.
Admiral of the Navy was only ever held by one US Officer; in 1945, the U.S. Navy considered creating the equivalent rank of Flag Admiral, but nothing came of the proposal.