A rough estimate of the time will do. To refer to mortal sins, the Catechism of the Catholic Church uses peccatum mortale (CCC 1856-1857), as does Saint Thomas Aquinas (ST II-I Q88 cf. New Advent). However, the Code of Canon Law refers to peccati gravis (Can 916). These are translated mortal sin and grave sin, respectively.
In my experience, many people will make a distinction between grave sin, which is a sin of grave matter, and mortal sin, which is a sin of grave matter carried out with full knowledge and intention. But St. Thomas says in the same question cited above, article 6:
Nevertheless a sin which is generically mortal, can become venial by reason of the imperfection of the act, because then it does not completely fulfil the conditions of a moral act, since it is not a deliberate, but a sudden act, as is evident from what we have said above (Article 2). This happens by a kind of subtraction, namely, of deliberate reason. And since a moral act takes its species from deliberate reason, the result is that by such a subtraction the species of the act is destroyed.
This seems to me to indicate that he thought mortal sin just referred to any sin which constituted grave matter, and that those sins could be made venial instead of mortal because of a lack of knowledge or intention, any circumstance which provides "subtraction... of deliberate reason."
Because of this, I'm suspicious of the term "peccatum gravis,” or "grave sin." I suspect it is a modernist invention meant to muddy the waters on sin and confuse the faithful into thinking that they can generally still be saved when in habitual mortal sin. This is verified when I hear anecdotes about priests telling penitents that they may receive communion prior to confession of "grave sins" because those sins are habitual, and thus not really mortal sins. Of course, even this advice appears to contradict the plain words of canon law, which appears to me as even further evidence that this distinction was introduced in order to confuse.
Does any pre-modern orthodox theologian, especially a Doctor of the Church, use the term "peccatum gravis?" And do they use it in this manner?