The Collaborative International Dictionary of English v.0.48:

Bisexual \Bi*sex"u*al\, a. [Pref. bi- + sexual.] (Biol.)
   Of both sexes; hermaphrodite; as a flower with stamens and
   pistil, or an animal having ovaries and testes.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bisexual
    adj 1: sexually attracted to both sexes [ant: heterosexual,
           homosexual]
    2: having an ambiguous sexual identity [syn: bisexual,
       epicene]
    n 1: a person who is sexually attracted to both sexes [syn:
         bisexual, bisexual person]