Mirriam-Webster's
Definition of SUPERNATURAL
1: of or relating to an order of existence beyond the visible observable universe; especially : of or relating to God or a god, demigod, spirit, or devil
2a : departing from what is usual or normal especially so as to appear to transcend the laws of nature b : attributed to an invisible agent (as a ghost or spirit)
It does fit the bill for the first part of the definition in that it is talking about something beyond the "visible observable universe". To go beyond that we are definitely into the speculative.