A flagship is a vessel used by the commanding officer of a group of naval ships, reflecting the custom of its commander, characteristically a flag officer, flying a distinguishing flag. Used more loosely, it is the lead ship in a fleet of vessels, typically the first, largest, fastest, most heavily armed, or, in terms of media coverage, best known.

Over the years the term "flagship" has been borrowed in metaphoric form by industries such as broadcasting, automobiles, education, and retailing to refer to their highest profile or most expensive products and outlets.

In common naval use the term "flagship" is fundamentally a temporary designation; the flagship is wherever the admiral's flag is being flown. However, admirals have always needed additional facilities; a meeting room large enough to hold all the captains of the fleet, and a place for the admiral's staff to make plans and draw up orders.

In the age of sailing ships, the flagship was typically a first-rate; the aft of one of the three decks would become the admiral's quarters and staff offices. This can be seen today on HMS Victory, the flagship of Admiral Nelson at the Battle of Trafalgar, now at Portsmouth, England. HMS Victory still serves the Royal Navy today as the ceremonial flagship of the Commander-in-Chief Naval Home Command making her the oldest commissioned warship in service. The USS Constitution, which also served as flagship of the United States Navy in her day, is 30 years younger, but is the oldest commissioned warship still afloat.