"[...] myth itself is neither fact nor fiction. Myth is a species of truth that precedes that distinction" (113.)
I think I've found mythology defined just as well as it is here, but I don't know if I can find it defined better. Incidentally, it reminds me of something from Northrop Frye's book, "The Great Code," which was one of my textbooks in my Bible class last term. Something discussed in chapter two is history; specifically Weltgeschichte, and Heilsgeschichte. Don't ask me how to pronounce those words - I never could figure it out exactly. All I know is that they're German. Anyways, Weltgeschichte is history as it literally happened. Heilsgeschichte, on the other hand, Frye defines as
"This may not be what you would have seen if you had been there, but what you would have seen would have missed the whole point of what was really going on" (48.)
I take back what I said before, about that first quote being the best definition for mythology. That quote, paired with the definition for Heilsgeschichte, is the best definition for mythology I've yet encountered.