From John Daly:
Correctly, the material element involved in being a heretic is conscious dissent from the Catholic rule of faith, while the formal element is the perverse state of the will which this entails. The distinction thus made, a Catholic who inculpably advances a heretical proposition by inadvertence may perhaps be said to have advanced a material heresy; but he cannot be called a material heretic. He is not a heretic in any sense. A heretic is one who dissents altogether from the Catholic rule of faith, and he will be called a material heretic if he is invincibly ignorant of the authority of the Church which he rejects, and a formal heretic if the Church's authority has been sufficiently proposed to him, so that his dissent from it is culpable. (This is clearly explained by Cardinal Billot: De Ecclesia Christi, ed. 4, pp. 289-290)
From the Catholic Encyclopedia:
Both matter and form of heresy admit of degrees which find expression in the following technical formula of theology and canon law. Pertinacious adhesion to a doctrine contradictory to a point of faith clearly defined by the Church is heresy pure and simple, heresy in the first degree. But if the doctrine in question has not been expressly "defined" or is not clearly proposed as an article of faith in the ordinary, authorized teaching of the Church, an opinion opposed to it is styled sententia haeresi proxima, that is, an opinion approaching heresy. Next, a doctrinal proposition, without directly contradicting a received dogma, may yet involve logical consequences at variance with revealed truth. Such a proposition is not heretical, it is a propositio theologice erronea, that is, erroneous in theology. Further, the opposition to an article of faith may not be strictly demonstrable, but only reach a certain degree of probability. In that case the doctrine is termed sententia de haeresi suspecta, haeresim sapiens; that is, an opinion suspected, or savouring, of heresy (see THEOLOGICAL CENSURES).