This is one of the better definitions of who a heretic is I've come across, from the late Fr Vitaly Borovoy:
In general a heretic is not a person who makes mistakes but is a Christian who puts his person interpretation of his beliefs agains the teaching of the Church when the Church has clearly expressed its judgment on his mistakes through Ecumenical Councils or by other appropriate means.
Taken from Does Chalcedon Divine Or Unite? Towards Convergence in Orthodox Christology, p. 125. Illustration is of St Nicholas striking Arius.