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.
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.
An offense against Christianity, consisting in a denial of some essential doctrine, which denial is publicly avowed, and obstinately maintained.
It is surely harmful to souls to make it a heresy to believe what is proved. Galileo Galilei
Neither was there any heresy, or diversity of opinion, or disputing about the matter, till the pope had gathered a council to confirm this transubstantiation: wherefore it is most likely that this opinion came up by them of latter days. William Tyndale