Source : Webster's Revised Unabridged Dictionary (1913)

Dauphin \Dau"phin\, n. [F. dauphin, prop., a dolphin, from L.
   delphinus. See {Dolphin}. The name was given, for some reason
   unexplained, to Guigo, count of Vienne, in the 12th century,
   and was borne by succeeding counts of Vienne. In 1349,
   Dauphiny was bequeathed to Philippe de Valois, king of
   France, on condition that the heir of the crown should always
   hold the title of Dauphin de Viennois.]
   The title of the eldest son of the king of France, and heir
   to the crown. Since the revolution of 1830, the title has
   been discontinued.

Source : WordNet®

dauphin
     n : formerly, the eldest son of the King of France and direct
         heir to the throne