From that bastion of knowledge, Wikipedia:
"Science fiction is largely based on writing rationally about alternative possible worlds or futures. It is related to, but different from fantasy in that, within the context of the story, its imaginary elements are largely possible within scientifically established or scientifically postulated physical laws (though some elements in a story might still be pure imaginative speculation).
The settings of science fiction are often contrary to those of consensus reality, but most science fiction relies on a considerable degree of suspension of disbelief, which is facilitated in the reader's mind by potential scientific explanations or solutions to various fictional elements. Science fiction elements include:
A time setting in the future, in alternative timelines, or in a historical past that contradicts known facts of history or the archaeological record.
A spatial setting or scenes in outer space (e.g. spaceflight), on other worlds, or on subterranean earth.
Characters that include aliens, mutants, androids, or humanoid robots and other types of characters arising from a future human evolution.
Futuristic or plausible technology such as ray guns, teleportation machines, and humanoid computers.
Scientific principles that are new or that contradict accepted physical laws, for example time travel, wormholes, or faster-than-light travel or communication.
New and different political or social systems, e.g. utopian, dystopian, post-scarcity, or post-apocalyptic.
Paranormal abilities such as mind control, telepathy, telekinesis (e.g. "The Force" in Star Wars).
Other universes or dimensions and travel between them."