What is Admiral?

Legal Definition
Admiral is the rank, or part of the name of the ranks, of the highest naval officers. It is usually abbreviated to "Adm" or "ADM".

The rank is generally thought to have originated in Sicily from a conflation of Arabic: أمير البحر‎‎, amīr al-baḥr, "commander of the sea", with Latin admirabilis ("admirable") or admiratus ("admired"), although alternative etymologies derive the word directly from Latin, or from the Turkish military and naval rank miralay.

In the Commonwealth and the U.S., a "full" admiral is equivalent to a "full" general in the army, and is above vice admiral and below admiral of the fleet (or fleet admiral). In NATO, admirals have a rank code of OF-9 as a four-star rank.
-- Wikipedia
Legal Definition
Officer. In some countries is the commander in chief of the naval forces. This office does not exist in the United States.
-- Bouviers Law Dictionary
Legal Definition
In European law. An officer who presided over the admiralitas or collegium ammiralitatis. Loce, de Jur. Mar. lib. 2, c. 2, § 1. In old English law. A high officer or magistrate that had the government of the king's navy, and the hearing of all causes belonging to the sea. Cowell. In the navy. Admiral is also the title of high naval officers; they are of various grades,—rear admiral, vice-admiral, admiral, admiral of the fleet, the latter being the highest
-- Black's Law Dictionary