heresy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An opinion or a doctrine at variance with established religious beliefs, especially dissension from or denial of Roman Catholic dogma by a professed believer or baptized church member.
• noun Adherence to such dissenting opinion or doctrine.
• noun A controversial or unorthodox opinion or doctrine, as in politics, philosophy, or science.
• noun Adherence to such controversial or unorthodox opinion.

from The Century Dictionary.

• noun Any doctrine, opinion, or set of opinions at variance with the established standards of any system, school of thought, or party; an opinion or a doctrine tending to create schism or division; an untenable or a disturbing doctrine of any kind, as in philosophy, science, politics, morality, etc.
• noun Specifically, in theology, an opinion or a doctrine rejected by the authorities of a church as contrary to the established creed of that church; an interpretation or a theological view of a sacred writing or other standard of religion, or of any distinctive part of it, opposed to that authoritatively established or generally accepted: as, the antinomian heresy.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• noun (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.
• noun (Law) An offense against Christianity, consisting in a denial of some essential doctrine, which denial is publicly avowed, and obstinately maintained.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun religion A doctrine held by a member of a religion at variance with established religious beliefs, especially dissension from Roman Catholic dogma.
• noun A controversial or unorthodox opinion held by a member of a group, as in politics, philosophy or science.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a belief that rejects the orthodox tenets of a religion
• noun any opinions or doctrines at variance with the official or orthodox position