Source : Webster's Revised Unabridged Dictionary (1913)

Princess \Prin"cess\, n. [F. princesse. See {Prince}, and cf.
   {Princesse}.]
   1. A female prince; a woman having sovereign power, or the
      rank of a prince. --Dryden.

            So excellent a princess as the present queen.
                                                  --Swift.

   2. The daughter of a sovereign; a female member of a royal
      family. --Shak.

   3. The consort of a prince; as, the princess of Wales.

   {Princess royal}, the eldest daughter of a sovereign.

Source : WordNet®

princess royal
     n : the eldest daughter of a British sovereign