Any traditional Catholic knows that there are two types of sin, viz. mortal and venial.  Mortal sin causes one to lose the state of grace and be worthy of hell, whereas venial sin does not.  Everyone here should be well aware of the three conditions, so I won't belabor those here.

Terms such as "grave" and "serious" sin are confusing and, as I see it, open the door to considering that there is a continuum, beginning with "venial", then moving on to "serious", then "grave", and then "mortal", in such a way that modern Catholics seem to be sliding into a concept of "grave but not mortal".  Examples include contraception, self-gratification, and even premarital sex and cohabitation, with the more daring of these people advocating for ɧoɱosɛҳųαƖ relationships.   If I had to crawl into the heads of these people, I have to imagine that they'd say something like "yes, these sins are grave, but they're not mortal, even if they are committed with sufficient reflection and full consent of the will, they stop something short of mortal, and thus such sinners may freely receive communion".  Not sure how they deal with confession.  Do they say "yes, this is grave, but I do not repent of it, because I have to keep committing this sin"?

Thoughts?

I think "grave" refers to the matter of the sin. Grave matter plus the other two conditions  = "mortal".

Some people may use "grave" as a synonym for "mortal".

If pushed, I think "serious" means an intentional or willful venial sin.

 I know that even pre-Vatican II (Jone et al), moral theology manuals and other works referred to "grave" sin, or in the case of Ott, "grievous" sin, and one should keep in mind, both Jone and Ott were translated from German.  So there is really nothing "wrong" with the word, if it can be seen as a synonym for "mortally sinful in and of itself", i.e., a mortal sin given the other two conditions.  "Serious" sin is a bit more ambiguous.  I really do have the suspicion that modern writers and others want to muddy the waters, the end goal being to carve out some space between venial and mortal, and call it "grave" or "serious".  That would, of course, be a misuse of the terms, but it could mollify those "in the mushy middle", who aren't quite able to say the Church is "wrong" to label some sins as grave, but want a kind of "wiggle room" to hold out hope that even "grave" sinners are not lost.  In the end, they basically want to limit "mortal" sin to such things as the popular consensus about men such as Hitler or Stalin.  They want to suggest, if only subliminally, that basically everyone is saved, even if they die in unrepented "grave" or "serious" sin.

I prefer to use the phrase "mortally sinful in and of itself" rather than "grave".  It leaves no room for interpretation or ambiguity.