The Collaborative International Dictionary of English v.0.48:

Marcionite \Mar"cion*ite\ (m[aum]r"sh[u^]n*[imac]t), n. (Eccl.
   Hist)
   A follower of Marcion, a Gnostic of the second century, who
   adopted the Oriental notion of the two conflicting
   principles, and imagined that between them there existed a
   third power, neither wholly good nor evil, the Creator of the
   world and of man, and the God of the Jewish dispensation.
   --Brande & C.
   [1913 Webster]