Slipstream is literature that has fantastic elements that nevertheless can't be characterized as science fiction, fantasy or horror. It is the literature of the fantastic that's left after you remove the defining characteristics of those three genres. The negative space.
Synopsis from ArmadilloCon program book. A plethora of new markets for what we used to call slipstream has blossomed over the past two years, with all sorts of new monikers like "The New Fabulists," "Interstitial Arts," and "Ambient Fiction" -- Conjunctions 39, Sweeney's Mammoth Treasury of Thrilling Tales (with a new volume appearing in the fall), Polyphony, Album Zutique, Lady Churchill's Rosebud Wristlet, and so on. What distinguishes the work appearing in these journals from more conventional SF, fantasy, and mainstream fiction? Has slipstream finally arrived as a genre in its own right?