Real Dictionary

heresy
variable noun: Heresy is a belief or action that most people think is wrong, because it disagrees with beliefs that are generally accepted.
variable noun: Heresy is a belief or action which seriously disagrees with the principles of a particular religion.

heresy in American English
a religious belief opposed to the orthodox doctrines of a church; esp., such a belief specifically denounced by the church
noun: opinion or doctrine at variance with the orthodox or accepted doctrine, esp. of a church or religious system

heresy in British English
noun: an opinion or doctrine contrary to the orthodox tenets of a religious body or church