Now, here's how I came to this point. I love just about anything Sci-Fi or Fantasy, but I've always felt lumping movies like E.T. and Jurassic Park together in the same category is a disservice to both of these movies. Now, let's take a look...
E.T. is a movie about an alien that gets stranded on Earth and befriends a boy named Elliot who helps him to return home. Along the journey we learn that E.T. has these magical powers of telekinesis that have no explanation other than they're magic. Where as, Jurassic Park is a movie about a reserve on a remote island in the Pacific where scientists have genetically engineered dinosaurs in a modern day setting, but when the power goes out on the island and the dinosaurs escape their paddocks, chaos ensues in trying to escape.
Now, both are fantastic movies that I love dearly, but the distinction comes in that E.T. works without any explanation of the scientific elements, and Jurassic Park can't work without them. E.T. can make a bike fly, we don't have to know how, we just have to know he can. Where on the flip side, Jurassic Park would not work without that scientific component of how the scientists genetically engineered these dinosaurs, cause otherwise the audience would never be able to suspend disbelief that there are dinosaurs in a modern day setting. Had the moviemaker (Steven Spielberg, on both counts) decided to say E.T. can make a bike fly cause he has this special chromosome in his body that allows him to manipulate matter, or that these scientists just happened upon an island full of dinosaurs, you would not only be stripping away the charm of these respective movies, but ultimately the suspension of disbelief would not be there.
So what is Science Fantasy? Science Fantasy (which is a term I'm coining myself, though I'll never copyright it) is any movie that has a Science Fiction component, like an alien or a time machine, but it is presented more like a Fantasy. Examples of this are E.T., where E.T.'s powers can simply be described as magic; or Star Wars, where the Force is a spiritual entity that enables those highly attuned to use it in mystical ways, and yet the mechanics of the spaceships and blasters are never explained. Other quick and dirty examples, are: Close Encounters of the Third Kind, The Time Machine, and even something like Superman: The Movie.
So what is Scientific Fiction (another term I'm coining, but wont copyright)? Scientific Fiction is any movie that needs that scientific explanation of the Sci-Fi element, whether it be aliens or time travel, in order for that movie to work. Examples of this are Jurassic Park and its genetically engineered dinosaurs, or Back to the Future and the flux capacitor, "That makes time travel possible." Other examples, are: Avatar, District 9, and 2001: A Space Odyssey.
As you can see for yourself, there is a clear distinction. So is it an injustice to simply lump every movie that's ever featured an alien or time travel into Science Fiction? I believe so. While a movie may have an alien featured prominently in its story, it may not be about aliens at all, but rather about the relationship between an odd creature -- that could just as easily have been Bigfoot -- and a little boy, or about a man searching for the spiritual meaning behind these lights he's seeing in the sky. Where as, you watch a movie like Back to the Future and Jurassic Park cause you wanna see how the moviemakers pull off the conventions presented. How does Marty go back in time? How do these modern day scientists come across an island full of dinosars? You have to have a "How" with Scientific Fiction, or else the viewing is damaged.
While I have a personal love for both kinds of Sci-Fi, I have always found myself more partial to Science Fantasy, rather than Scientific Fiction. Maybe it's my own hatred of science, or my own love of the fantastical and the unexplained that makes me like Science Fantasy movies just a teensy bit more, but whatever it is, I don't think you can look at a Science Fiction movie the same way again after giving this a little bit of thought.