Jack

1. A sailor. Also jack tar or just tar.
2. (jack flag) A national or other official flag flown on a short jackstaff at the bow of a vessel indicating nationality or subordination to a navy or other particular seagoing service or to a government department or subnational government (such as a state or province), or to indicate membership in a yacht club. Typically, crew members spoke of the jack as if it were a member of the crew. A jack contrasts with an ensign, which is a flag with a generally similar purpose flown from the vessel′s stern. Typically, vessels fly a jack while in port and an ensign while at sea (in daylight hours).
3. Informally, any flag flown by a ship.