The ship occupied by the fleet's commander (usually an admiral); it denotes this by flying his flag.
The most important one out of a related group.
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.
The highest grade of flag officer in the United States Navy, ranking above a four-star admiral and junior only to the Secretary of the Navy, Secretary of Defense, and the President of the United States; equal in grade or rank to a general of the army. Usually called admiral of the fleet in other navies.