A definition offered by
Rod Serling holds that "science fiction, the improbable made possible; fantasy, the impossible made probable".
[1] The meaning is that science fiction describes unlikely things that
could possibly take place in the real world under certain conditions, while science fantasy gives a scientific veneer of realism to things that simply could not happen in the real world under any circumstances. Another interpretation is that science fiction does not permit the existence of
fantasy or
supernatural elements; science fantasy does.
For many users of the term, however, "science fantasy" is either a science fiction story that has drifted far enough from reality to "feel" like a fantasy, or a fantasy story that is attempting to be science fiction. While these are in theory classifiable as different approaches, and thus different genres (fantastic science fiction vs. scientific fantasy), the end products are sometimes indistinguishable.