means A nation's organized fleet of warships and the personnel who operate them, the branch of the armed forces that fights at sea.

from From the Old French 'navie,' meaning a fleet or ship, which traces back to the Latin 'navis,' simply 'ship.' That same 'navis' is the deep ancestor of 'navigate,' 'nave' (a church's long central hall, shaped a bit like an inverted hull), and even 'nausea' — by way of Greek 'naus,' a cousin of the Latin word, since seasickness was the original kind. The dark uniform color called 'navy blue' came much later, named for the standard dress of the British Royal Navy.