Usually the term “formal” is used vis-a-vis “material.”
The formal/material distinction has to do with intention. If I teach something contradictory to the Church’s teaching and I know it’s contradictory to the Church’s teaching, that’s formal heresy (well, if I’ve been officially warned not to do it, anyway, otherwise it’s just false teaching). If I teach something contradictory to the Church’s teaching, but I think that what I’m teaching is the Church’s teaching, then that’s material heresy.