Copyright by San Diego Writers
Speculative Fiction deals with the what-if scenarios in all its possibilities. The earliest forms were probably mythological tales being told around the campfire. Today you'll find stories that take you on journeys to other worlds or on magical quests, people will have supernatural powers or be able to travel through time. While all of these fall into individual genres, often crossing borders between those genres too, they have one thing in common - they are Speculative Fiction.
In its English language usage in arts and literature since 20th century, the genre term is often attributed to Robert A. Heinlein though he used it specifically as a synonym for Science Fiction, and explicitly stated that his use of the term did not include fantasy. The use of Speculative Fiction to show dissatisfaction with traditional Science Fiction was popularized in the 1960s and early 1970s by Judith Merril though it fell in disuse soon thereafter. In the 2000s, the term has come into wider use as a convenient collective term for a whole set of genres.
To me it seems that the category of Speculative Fiction opens up whole new dimensions especially where just one designated genre, eg. Science Fiction, simply does the book no justice. How often have you marveled what genre a certain book exactly falls into? More than just once, right? And you're not alone with that. Maybe calling many novels out there Speculative Fiction is taking the easy route of not having to put it where it's not fully at home.
And, as an afterthought, some authors don't want to be pigeonholed solely as a Science Fiction writer, thus declaring themselves at home in the realms of Speculative Fiction. Well, one of those authors might be me ... but that's just because what I write can't be pigeonholed either. Some of my stories are clearly Science Fiction, but others cross over this genre's border in all kinds of directions. It almost looks like the term Speculative Fiction has been invented for me.
Tid-bit for the road: The term Suppositional Fiction is sometimes used as a sub-category designating fiction in which characters and stories are constrained by an internally consistent world, but not necessarily one defined by any particular genre.