Science Fantasy is one of the harder Setting Aesthetics to describe. Abstracted into one single sentence, it is a setting including elements from both traditional fantasy and science-fiction. It can take many forms, for science fantasy spans the whole gamut between "Hard Science-Fiction, but something like psionics/magics/... that cannot be explained by science exists" to "Fantasy, but where advanced technology do exist as is mistaken for magic".
Science fantasy dates all the way back to the origins of fantasy as a whole. See, despite modern "purist wars", science fiction and fantasy weren't considered really distinct from each other in the early days. As a result, you had lots of things like magical aliens, barbarians wielding laser pistols as they rode across post-apocalyptic worlds, time traveling sorcerers, and so forth. (The most telling example is Conan the Barbarian, archetypal Fantasy hero, encountering space-faring travellers in his adventures.) It's a very fertile aesthetic, and is not going to be going anywhere any time soon.
Science Fiction vs. Science FantasyEdit
Its worth pointing out that there are settings which technically qualify as science fantasy but are generally not considered one due to the fantasy aspects being used very sparingly. Star Trek is probably the most notable example of this, as it does have things such as Q and the Prophets but they are rarely featured, hence why Star Trek is generally considered to be just science-fiction.
It also helps that such intrusions of Fantasy elements are treated in a true Science Fiction-y way: Q and similar "Godlike" entities are viewed as merely a "sufficiently advanced Alien" in the Arthur C. Clarke sense (and we are shown the flipside, how Picard & Crew look like Gods to sufficiently primitive aliens, in a couple of ST:TNG episodes), and the Prophets are mainly used as a method to have time-related shenanigans, or have some religion vs. science debate where the divine entity (A) explicitly exists, (B) is explainable as "sufficiently advanced and unusual aliens", and (C) aren't jerks, just bad at communication with those of us who experience time linearly.
Dune provides another good example: While there are plenty of mystical and "magical" elements, they are examined the same as any other natural phenomena; the religious elements are used purely for in-universe-manipulation, not treated as in any way true.
Star Wars, on the other side of the line, explicitly has mystical elements without explanation that are treated as mystical both in-universe and by the authors.
This is edifying for the dividing line between Science Fantasy and Science Fiction: How the "magical" elements are treated is usually more important than what, exactly, those elements are in determining what genre you are in.
Examples of Science Fantasy on /tg/Edit
Star Wars: Probably the best known "Science Fantasy" setting. While it originates from outside tabletop gaming, there have been vast numbers of roleplaying, miniature and board games set in Star Wars. Indeed, the West End Games RPG laid the foundation for everything that wasn't in the films or Lucas's notes.
Superhero Settings: While individual heroes can be relatively pure Science Fiction, Horror, or Urban Fantasy, when you start glomming them together and try to make it make sense, you usually wind up in Science Fantasy pretty easily. (Yes, it still counts as /tg/; Mutants & Masterminds, Champions, Villains and Vigilantes, Superworld, dedicated Splat for generic systems like GURPS or Savage Worlds, and over a dozen licensed games based on DC and Marvel all exist, and that non-exhaustive list doesn't even get into systems based on Japanese style super heroes.)
Numenera: A world presented in classic fantasy style... except it's actually set on an Earth billions of years into the future; all of the "magic items" are actually hyperscience devices whose underlying principles have been forgotten, spellcasters are either using psionics or hypertech gizmos, and the various non-humans and monsters are aliens from other worlds/dimensions, genetically engineered lifeforms, or robots.
Warhammer 40,000: Literally beginning its existence as "Warhammer Fantasy IN SPACE!", 40K is a classic example of the "tech first, fantasy second"( or is it "fantasy first, tech second" ?) aproach to Science Fantasy. The Space Marines are techno-knights cum paladins, the Eldar are spacefaring elves, Dark Eldar tap into all sorts of "nasty faerie boogeyman" archetypes, psionics is essentially magic with sci-fi trappings, the orks are literally just orcs with guns and spaceships...
Blackmoor: Is a Dungeons & Dragons setting in a world full of advanced science, and it's canonically the pre-apocalyptic past of Mystara. It's also the setting for the adventure modules "Temple of the Frog" and "City of the Gods".
Torg: Original in the sense it allowed multiple "realities" to co-exist (and duke it out) on Earth, so your average party could very well consist of a clusterfuck of a cyborg Hulk, a prehistoric era shaman with his stone-tipped spear summoning a dinosaur to ride in battle, a suave Indiana Jones-esque adventurer and a ninja using both supernatural ninja powers and a machinegun.
RIFTS: Just... where do we start with RIFTS? To quote our frenemies on TVtropes: The tabletop RPG Rifts is set a few centuries after the high tech world of tomorrow is utterly trashed by the return of magic. Human supremacist armies of cyborgs, chemically-enhanced supersoldiers and Humongous Mecha traipse across the landscape. Atlantis has risen. Sorcerers summon demons and raise the dead. Rifts in spacetime spew out critters from other dimensions more or less at random. Elves and dragons and goblins (oh my) roam the wilderness. Killer cyborgs from another dimension want to kill all humanoid life on Earth. Gods battle Alien invaders. Vampires openly run entire cities. And that's just the tip of the iceberg.
Shadowrun: A world where The Magic Comes Back meets cyberpunk, so you've got dragons running megacorporations, elves with laser pistols, dwarves building killer robots and cyborg orcs & trolls all duking it out on the mean streets.
Pathfinder: Shamelessly using this trope, Pathfinder has everything from a "crashed alien spaceship in a land of barbarians" region on the planet itself to sourcebooks detailing the other worlds in the solar system, so you can involve interplanetary travel as much as you like. Its successor/spin-off Starfinder is basically Pathfinder set 1000 years later, so everyone lives in space after Golarion vanished.
Starfinder: AKA "Pathfinder IN SPACE". More Science Fiction to Pathfinder's High Fantasy, and thus even further into Science Fantasy.
Spelljammer: A Dungeons & Dragons campaign setting that revolves around using magical ships (and by ships we mean "sail the seven seas" type) to fly between "crystal spheres" (fantasy solar systems) by way of an alternate dimension called the Phlogiston.
Eberron: Another D&D campaign setting. Eberron is based in "wide magic", where sufficiently understood magic is indistinguishable from technology. While the baseline mixes pulp of the Industrial Revolution and the Interwar, many applications wouldn't be out of place in science fiction: Instead of sinks, bathrooms have devices that magically clean the user's everything, portals to the moons (and perhaps even beyond) are possible, artificial limbs of both the magical and biological alien variety exist and two types of aliens seek to invade it.
Draenei: A race from World of Warcraft who originated on an alien planet, were rescued from demonic invaders by angel-like beings made of divine magical energy, and traveled to Azeroth in gargantuan fortresses magically propelled between worlds.