Not To Be Confused With
Classically a heretic is not merely someone who is mistaken, but someone who persists in denying a defined essential after being corrected. Honest inquirers and the simply confused are not heretics.
The historic distinction is between material and formal heresy: holding a wrong belief in ignorance (material) versus knowingly and obstinately rejecting the Church's defined teaching (formal). Only the latter made one a “heretic” in the full sense.
The word has often been thrown as an insult; used carefully it is a precise, serious term, not a label for everyone outside one's own camp.