NA meaning in Military and Defence ?

A naval academy is a national institution that provides undergraduate-level education for prospective naval officers.

Naval training commonly took place only at sea until the 19th century, even if those ships were permanently moored. The Royal Naval Academy shoreside facility established in 1733 was closed in favor of shipboard officer training in 1837. The United States Naval Academy began in 1845. Yet both the French École Navale and the British BRNC only comparatively recently constructed their training establishments on shore (1914 and 1905, respectively), despite the history of those two navies – almost contemporary with the establishment of those two nations air force colleges. This is in contrast to their military counterparts such as the École Spéciale Militaire de Saint-Cyr and the Royal Military Academy Sandhurst which, with their predecessors, have a history of a much more formal presence, in spite (as in the British case) of the Royal Navy being the service that was both 'senior' and historically had commanded most of the state funds for defence purposes.

Some states may combine all officer training for their armed forces into one college. This is particularly true where economies of scale mean that it is impractical to have separate naval, military and air force academies. It may also be the case in those few of the world's states that have unified armed forces – the most notable one being the Canadian Forces.