Sort of. Obscenity is relative; it's something that offends the prevalent morality of the time and thus what is obscene differs by both time and place. Or for a related definition it must be material "utterly without redeeming social importance". Of course, what is "socially important" is very much changable and ill-defined.
As a result, the question of what is and what isn't rarely easily answerable. In the US, these days, child porn is obscene (but is also illegal several times over via other laws, so that hardly matters) and even a textual and fictional account of the rape of a child might be obscene if it seems to a jury that it was meant purely to titillate. On the other hand, an autobiography of someone who was abused as a child, or a textbook meant for social workers discussing how to help the victim of child abuse would not be obscene (assuming, of course, a jury agrees on what the material in question is). Depictions of bestiality and other similar acts are probably obscene, but maybe not - it all depends on what a jury says. And there are different rules at the national level and in the various states. Sex toys are never obscene at the national level, but are obscene in Alabama. On the other hand, nothing is obscene in Oregon.
As a result, the question of what is and what isn't rarely easily answerable. In the US, these days, child porn is obscene (but is also illegal several times over via other laws, so that hardly matters) and even a textual and fictional account of the rape of a child might be obscene if it seems to a jury that it was meant purely to titillate. On the other hand, an autobiography of someone who was abused as a child, or a textbook meant for social workers discussing how to help the victim of child abuse would not be obscene (assuming, of course, a jury agrees on what the material in question is). Depictions of bestiality and other similar acts are probably obscene, but maybe not - it all depends on what a jury says. And there are different rules at the national level and in the various states. Sex toys are never obscene at the national level, but are obscene in Alabama. On the other hand, nothing is obscene in Oregon.
tl;dr summary: Has a border been defined? No. :)