The term lemon is, as far as I know, only used in anime/manga fanfiction. No one uses it in the Lord of the Rings or Harry Potter fandoms that I've seen.

Yaoi is another one only used in anime/manga.

I also have a friend who insists that any non-canon pairing (whether heterosexual or homosexual) is slash, but I don't know what fandom that would apply in. I've never seen it used that way.