an admiral of the highest rank; five-star admiral

Admiral of the Fleet (Royal Navy)

A Fleet Admiral or Admiral of the Fleet, as it was first coined, is a military naval officer of very high rank, if not the highest. It is also a generic term for a senior Admiral in command of a large group of ships, comprising a fleet or, in some cases, a group of fleets. If actually a rank (equivalent to an army Field Marshal or General of the Army, which ranks above General), it's name varies depending on the country, including Fleet Admiral, Admiral of the Fleet, Admiral of the Navy, and Grand Admiral.

A Fleet Admiral in the United States Navy is an admiral considered to be the equivalent of the United States Army's General of the Army.

The rank of Fleet Admiral is common in many science fiction sources as a senior military officer who commands a fleet of spaceships and the rank has been mentioned in numerous films, television productions, and science fiction literature sources.