Speculative fiction is a broad genre of fiction with any supernatural, fantastical, or futuristic elements. Coined by Robert Heinlen, it’s considered an off-shoot of science-fiction, often applied to avoid the limitations of traditional “realistic” sci-fi. In more contemporary times, it’s applied to stories that straddle the line between science-fiction and fantasy.

As a side-note of some amusement, Heinlen insisted it should never be applied to fantasy, but only science-fiction. Of course, science-fantasy falls squarely under its umbrella, so that wish has gone unfulfilled.

Have a Wikipedia article.