The Collaborative International Dictionary of English v.0.48:

Birthright \Birth"right`\, n.
   Any right, privilege, or possession to which a person is
   entitled by birth, such as an estate descendible by law to an
   heir, or civil liberty under a free constitution; esp. the
   rights or inheritance of the first born.
   [1913 Webster]

         Lest there be any . . . profane person, as Esau, who
         for one morsel of meat sold his birthright. --Heb. xii.
                                                  16.
   [1913 Webster]

WordNet (r) 3.0 (2006):

birthright
    n 1: a right or privilege that you are entitled to at birth;
         "free public education is the birthright of every American
         child"
    2: an inheritance coming by right of birth (especially by
       primogeniture) [syn: birthright, patrimony]
    3: personal characteristics that are inherited at birth