Another way to describe science fiction would be to say that the stories involve, either directly or indirectly, plausible fictional science.

"Fictional science" is real or imagined science that we don't know about yet.  If Jules Verne had written that the Nautilus in "20,000 Leagues Under the Sea" was powered by nuclear fission, the story would have been based on real, but unknown, science.  Warp drive, in Star Trek, is based on science that we don't know yet - we don't even know if it is real or just imagined science.

Fictional science can be an extropolation of the facts that we do know (or that we think we know).

If fictional science contradicts "known" science, then there must be some sort of plausible explanation to account for the discrepancy.

It isn't that the story involves science fact that makes it science fiction (if it did, then "House M.D." would be science fiction), but that it involves plausible fictional science.