From a naturalistic perspective, it is possible to argue that the supernatural not only doesn't exist, but cannot even be defined. The reasoning goes that anything which "appears" to be supernatural, is simply misunderstood or conforms to laws of nature of which humans are not yet aware.
Is this line of reasoning sound and there is only the natural, or can the supernatural be properly defined?
Traditional instances of supernatural acts and beings would include God/god(s), angels/devils, miracles, vampires/werewolves, magic spells, ghosts/spirits, etc.
Edit based on comments: The question was intended to illicit answers that argue for or against the possibility of defining what supernatural is, especially with regard to a backdrop of naturalism (vs. dualism, I suppose?). In my mind it is implicit that a good/proper definition would include testability in some fashion.