The Collaborative International Dictionary of English v.0.48:

Heretic \Her"e*tic\, n. [L. haereticus, Gr. ? able to choose,
   heretical, fr. ? to take, choose: cf. F. h['e]r['e]tique. See
   Heresy.]
   1. One who holds to a heresy; one who believes some doctrine
      contrary to the established faith or prevailing religion.
      [1913 Webster]

            A man that is an heretic, after the first and second
            admonition, reject.                   --Titus iii.
                                                  10.
      [1913 Webster]

   2. (R. C. Ch.) One who having made a profession of Christian
      belief, deliberately and pertinaciously refuses to believe
      one or more of the articles of faith "determined by the
      authority of the universal church." --Addis & Arnold.

   Syn: Heretic, Schismatic, Sectarian.

   Usage: A heretic is one whose errors are doctrinal, and
          usually of a malignant character, tending to subvert
          the true faith. A schismatic is one who creates a
          schism, or division in the church, on points of faith,
          discipline, practice, etc., usually for the sake of
          personal aggrandizement. A sectarian is one who
          originates or is an ardent adherent and advocate of a
          sect, or distinct organization, which separates from
          the main body of believers.
          [1913 Webster]

WordNet (r) 3.0 (2006):

heretic
    n 1: a person who holds religious beliefs in conflict with the
         dogma of the Roman Catholic Church [syn: heretic,
         misbeliever, religious outcast]
    2: a person who holds unorthodox opinions in any field (not
       merely religion)

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "heretic":
   Albigensian, Arian, Bohemian, Cathar, Donatist, Ebionite, Erastian,
   Gnostic, Jansenist, Lollard, Manichaean, Monophysite, Montanist,
   Pelagian, Sabellian, Waldensian, Wyclifite, antinomian, apostate,
   beatnik, defector, deviant, deviationist, dissenter, dissident,
   dropout, emanationist, flower child, freak, hippie, hylotheist,
   iconoclast, infidel, maverick, misbeliever, misfit, nonconformist,
   nonjuror, original, pantheist, recreant, recusant, renegade,
   revisionist, schismatic, sectarian, sectary, separatist, swinger,
   ugly duckling, unbeliever, unconformist, yippie