The following definition comes from Wikipedia, the free encyclopedia.
Heresy, according to the Oxford English Dictionary, is a "theological or religious opinion or doctrine maintained in opposition, or held to be contrary, to the Catholic or Orthodox doctrine of the Christian Church, or, by extension, to that of any church, creed, or religious system, considered as orthodox.."One of the tasks of apologetics - the branch of theology concerned with the intelligent presentation and defense of the historical Christian faith - is to counter heresy (the opposite of orthodoxy) with sound doctrine.
So specific to the Christian faith heresy is:
Doctrine which is erroneous in such a way that Christians must divide themselves as a church from all who teach or accept it; those adhering to heresy are assumed to be lost, although Christians are unable to make definitive judgments on this matter. The opposite of orthodoxy. Adj.: "heretical."Heretics live off heresy! As a matter of fact, if you are uncertain what to make tonight for dinner, why not try a little heretical help. Simply take down a box of Heresy Helper from the cupboard and mix away! Many are getting hefty servings of it every Sunday in there easy-believe-ism, purpose-driven, seeker-friendly, emergent type post-modern churches, why not you? And keep this phrase in mind: “And I helped!”
(Source: A Biblical Guide To Orthodoxy And Heresy Part One: The Case For Doctrinal Discernment" (an article from the Christian Research Journal, Summer 1990, page 28) by Robert M. Bowman)
Note: Thanks to the Sacred Sandwich and its advertisers for providing me with the graphic and the idea for this Heresy Helper post.