Admiral of the Fleet John Arbuthnot Fisher, 1st Baron Fisher,, commonly known as Jacky or Jackie Fisher, was a British Admiral of the Fleet. With more than sixty years in the Royal Navy, his efforts to reform the service helped to usher in an era of modernisation which saw the supersession of wooden sailing ships armed with muzzle-loading cannon by steel-hulled battlecruisers, submarines and the first aircraft carriers.

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.