The Collaborative International Dictionary of English v.0.48:

Jansenist \Jan"sen*ist\, n. [F. Jans['e]niste.] (Eccl. Hist.)
   A follower of Cornelius Jansen, a Roman Catholic bishop of
   Ypres, in Flanders, in the 17th century, who taught certain
   doctrines denying free will and the possibility of resisting
   divine grace.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Jansenist
    n 1: an advocate of Jansenism