The Collaborative International Dictionary of English v.0.48:

Heresy \Her"e*sy\, n.; pl. Heresies. [OE. heresie, eresie, OF.
   heresie, iresie, F. h['e]r['e]sie, L. haeresis, Gr. ? a
   taking, a taking for one's self, choosing, a choice, a sect,
   a heresy, fr. ? to take, choose.]
   [1913 Webster]
   1. An opinion held in opposition to the established or
      commonly received doctrine, and tending to promote a
      division or party, as in politics, literature, philosophy,
      etc.; -- usually, but not necessarily, said in reproach.
      [1913 Webster]

            New opinions
            Divers and dangerous, which are heresies,
            And, not reformed, may prove pernicious. --Shak.
      [1913 Webster]

            After the study of philosophy began in Greece, and
            the philosophers, disagreeing amongst themselves,
            had started many questions . . . because every man
            took what opinion he pleased, each several opinion
            was called a heresy; which signified no more than a
            private opinion, without reference to truth or
            falsehood.                            --Hobbes.
      [1913 Webster]

   2. (Theol.) Religious opinion opposed to the authorized
      doctrinal standards of any particular church, especially
      when tending to promote schism or separation; lack of
      orthodox or sound belief; rejection of, or erroneous
      belief in regard to, some fundamental religious doctrine
      or truth; heterodoxy.
      [1913 Webster]

            Doubts 'mongst divines, and difference of texts,
            From whence arise diversity of sects,
            And hateful heresies by God abhor'd.  --Spenser.
      [1913 Webster]

            Deluded people! that do not consider that the
            greatest heresy in the world is a wicked life.
                                                  --Tillotson.
      [1913 Webster]

   3. (Law) An offense against Christianity, consisting in a
      denial of some essential doctrine, which denial is
      publicly avowed, and obstinately maintained.
      [1913 Webster]

            A second offense is that of heresy, which consists
            not in a total denial of Christianity, but of some
            its essential doctrines, publicly and obstinately
            avowed.                               --Blackstone.
      [1913 Webster]

   Note: "When I call dueling, and similar aberrations of honor,
         a moral heresy, I refer to the force of the Greek ?, as
         signifying a principle or opinion taken up by the will
         for the will's sake, as a proof or pledge to itself of
         its own power of self-determination, independent of all
         other motives." --Coleridge.
         [1913 Webster]

WordNet (r) 3.0 (2006):

heresy
    n 1: any opinions or doctrines at variance with the official or
         orthodox position [syn: unorthodoxy, heterodoxy,
         heresy] [ant: orthodoxy]
    2: a belief that rejects the orthodox tenets of a religion [syn:
       heresy, unorthodoxy]

Moby Thesaurus II by Grady Ward, 1.0:

107 Moby Thesaurus words for "heresy":
   Albigensianism, Arianism, Bohemianism, Catharism, Ebionitism,
   Erastianism, Gnosticism, Jovinianism, Lollardy, Manichaeanism,
   Manichaeism, Monophysism, Monophysitism, Pelagianism,
   Waldensianism, Wyclifism, aberrancy, aberration, agnosticism,
   ambiguity, ambivalence, antinomianism, antinomy, apostasy,
   asymmetry, atheism, beatnikism, defection, defectiveness, delusion,
   denial, deviancy, disbelief, discredit, disproportion,
   disproportionateness, dissent, dissidence, distortion, emanatism,
   equivocality, errancy, erroneousness, error, fallaciousness,
   fallacy, false doctrine, falseness, falsity, fault, faultiness,
   flaw, flawedness, fringiness, hamartia, heterodoxy, heterogeneity,
   hippiedom, hylotheism, illusion, impiety, inability to believe,
   incoherence, incommensurability, incompatibility, incongruity,
   inconsistency, inconsonance, incredulity, infidelity,
   irreconcilability, minimifidianism, misapplication, misbelief,
   misconstruction, misdoing, misfeasance, misinterpretation,
   misjudgment, nonbelief, nonconformability, nonconformism,
   nonconformity, nullifidianism, originality, oxymoron, pantheism,
   paradox, peccancy, perversion, rejection, revisionism, schism,
   self-contradiction, sin, sinfulness, unbelief, unbelievingness,
   unconformability, unconformity, unconventionality, unorthodoxy,
   untrueness, untruth, untruthfulness, wrong, wrongness



Easton's 1897 Bible Dictionary:

Heresy
   from a Greek word signifying (1) a choice, (2) the opinion
   chosen, and (3) the sect holding the opinion. In the Acts of the
   Apostles (5:17; 15:5; 24:5, 14; 26:5) it denotes a sect, without
   reference to its character. Elsewhere, however, in the New
   Testament it has a different meaning attached to it. Paul ranks
   "heresies" with crimes and seditions (Gal. 5:20). This word also
   denotes divisions or schisms in the church (1 Cor. 11:19). In
   Titus 3:10 a "heretical person" is one who follows his own
   self-willed "questions," and who is to be avoided. Heresies thus
   came to signify self-chosen doctrines not emanating from God (2
   Pet. 2:1).


Bouvier's Law Dictionary, Revised 6th Ed (1856):

HERESY, Eng. law. The adoption of any erroneous religious tenet, not
warranted by the established church.
     2. This is punished by the deprivation of certain civil rights, and by
fine and imprisonment. 1 East, P. C. 4.
     3. In other countries than England, by heresy is meant the profession,
by Christians, of religious opinions contrary to the dogmas approved by the
established church of the respective countries. For an account of the origin
and progress of the laws against heresy, see Giannoni's Istoria di Napoli,
vol. 3, pp, 250, 251, &c.
     4. in the United State, happily, we have no established religion; there
can, therefore, be no legal heresy. Vide Apostacy; Christianity.