A person, group, thing, or course of action that is powerful, influential, or particularly effective.
A large artillery gun or one of the largest-caliber naval guns (as on a battleship).
The most important one out of a related group.
The ship occupied by the fleet's commander (usually an admiral); it denotes this by flying his flag.
The ship regarded as most important out of a group, e.g. a nation's navy or company's fleet.
To act as a flagship for.