Source : Webster's Revised Unabridged Dictionary (1913)

Flagship \Flag"ship`\, n. (Naut.)
   The vessel which carries the commanding officer of a fleet or
   squadron and flies his distinctive flag or pennant.

Source : WordNet®

flagship
     n 1: the chief one of a related group; "it is their flagship
          newspaper"
     2: the ship that carries the commander of a fleet and flies his
        flag