USN is short for the United States Navy — the part of the American armed forces that fights at sea, with ships, submarines and aircraft that fly from ships. Like most military abbreviations it belongs to writing, not to speech: it appears on uniforms, on official papers, and after a person's name to show which service they belong to, as in 'Captain Ellen Ruiz, USN'. When people talk, they say 'the Navy'. Each letter is said separately (U-S-N). You will also meet it beside its cousins USMC (the Marines) and USAF (the Air Force), and beside USS, which goes in front of a ship's name. Outside American contexts it is not much used, so write the full name once before using the letters.