The definition is simple: fiction possessing elements that aren’t feasible based on modern technology or ones that cannot be explained by modern science.
This includes magic, ray guns, the supernatural, premonitions, downloading your consciousness into a computer, AI, talking to ghosts, time travel, clockwork beings, alien life forms, ships that hop and skip around the universe, and just about anything that we don’t have a ready answer for.
Speculative fiction is a catch-all term. Instead of having to sub-divide science fiction from fantasy (or define its hybrid baby, science-fantasy), separate high fantasy from urban fantasy, to parse out magic-based systems from non-magic systems, and try to figure out just where “weird stories” end and slipstream begins, we can just say it’s speculative fiction.
Having a term to encompass everything makes sense: rarely have I found a fan of science fiction and fantasy who will only read a certain subset of the genre. Genre readers read, read a lot, and read across boundaries. The bookstores understand this. They don’t keep the science fiction away from the fantasy; they’re all mixed up on the same shelves.
Surprisingly, I’ve heard people disparage the use of the term “speculative.”
Some of these people believe that the term is an attempt to sanitize genre fiction of its less-than-highbrow historical association with the pulp press, or that it’s an attempt to bring sci-fi into the mainstream. Others believe that speculative fiction is yet another subset of sci-fi.
Both groups are wrong.
Speculative fiction is not a disparaging, belittling, or sub-dividing term. It is a catch-all. A short cut. One that we can put to good use.