"...this is a kind of writing which simply makes you feel very strange; the way that living in the twentieth century makes you feel, if you are a person of a certain sensibility."—Bruce Sterling, in the SF Eye article coining the term, July 1989
Sometimes, stories crop up where there are no overt fantasy or science-fiction elements, yet odd stuff is still happening. Perhaps the hero wakes one day to find himself transformed into a gigantic insect or that he's the main character of someone else's novel, and the why of the incident never comes up. Or maybe the lines between fantasy and reality have become so blurred that it's hard to tell one from the other. Or maybe, the story is just so odd that the only thing you can think about after reading it is that point directly behind your head.
That is the genre (or possibly the literary device) known as Slipstream.
Originally coined by Cyberpunk author Bruce Sterling, Slipstream is often referred to as "the fiction of strangeness," and that's about as clear a definition as you can get. It falls somewhere between Speculative Fiction and mainstream or Lit Fic, depending on the work. Above all, Slipstream is about a feeling of surreality.