A first-rate ship of the line.
Of a Royal Navy ship of the line in the Napoleonic Era: having at least 100 guns across three gun decks, a complement of 850–875, and weighing approximately 2,500 tons burthen.
Superb, exceptional; of the best sort; very high quality.
The ship regarded as most important out of a group, e.g. a nation's navy or company's fleet.
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.
To act as a flagship for.