The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            So excellent a princess as the present queen.
                                                  --Swift.
      [1913 Webster]

   2. The daughter of a sovereign; a female member of a royal
      family. --Shak.
      [1913 Webster]

   3. The consort of a prince; as, the princess of Wales.
      [1913 Webster]

   Princess royal, the eldest daughter of a sovereign.
      [1913 Webster]

WordNet (r) 3.0 (2006):

princess royal
    n 1: the eldest daughter of a British sovereign