Board Thread:Naruto Discussions/@comment-3568954-20140820074807/@comment-1512913-20140826152034

There are two kinds of immortality:
 * one real, biological immortality
 * the other fictitious, spiritual immortality or whatever

There are both in Naruto. The closest thing to spiritual immortal was Hidan... ironically, he wasn't biologically immortal to compensate, so he has died most likely by now anyway, due to lack of nutrition and whatnot.

We don't know if Kaguya has the latter form as well. Maybe severing her head would do, maybe not.