Admiral of the Fleet is a five-star naval officer rank and the highest rank of the Royal Navy, formally established in 1688. The five-star NATO rank code is OF-10, equivalent to a field marshal in the British Army or a Marshal of the Royal Air Force. Apart from honorary appointments, no new admirals of the fleet have been named since 1995, and no honorary appointments have been made since 2014.

A five-star rank is the highest military rank in many countries. The rank is that of the most senior operational military commanders, and within NATO's standard rank scale it is designated by the code OF-10. Not all armed forces have such a rank, and in those that do the actual insignia of the five-star ranks may not contain five stars. For example: the insignia for the French OF-10 rank maréchal de France contains seven stars; the insignia for the Portuguese marechal contains four gold stars. The stars used on the various Commonwealth of Nations rank insignias are sometimes colloquially referred to as pips, but in fact either are stars of the orders of the Garter, Thistle or Bath or are Eversleigh stars, depending on the wearer's original regiment or corps, and are used in combination with other heraldic items, such as batons, crowns, swords or maple leaves.