Heresy is the perverting of the gospel of salvation. Unfortunately, this not the universally held definition. Many brand as heresy and split Churches over differences of opinion on what God will do and how he will do it. The Rapture comes to mind. I firmly believe that the Rapture or Translation of the Church prior to the Tribulation is a biblical fact, yet my closest brother and friend does not. We have no trouble fellowshiping and spend hours discussing the Scriptures.
My definition is very narrow and confined to perverting the gospel of salvation in some form or other as teaching works to obtain, maintain, or limit the scope of salvation to only some group or denomination.