To quote from the KurzweilAI.net website:

Wild Cards are low-probability, extremely high-impact events that are social and technological developments or natural phenomena sharing four characteristics. Wild Cards are:

* global in scope and directly effect the human condition;
* potentially disruptive (negatively and/or positively);
* intrinsically beyond the control of any single institution, group or individual; and
* rapidly moving.

Developing an understanding of such events (and trust me….there is lots of literature on the web about them. Everything from meteors hitting earth to global communication meltdowns) can help SF writers (and everyone else) explore the future.

Read more at http://www.kurzweilai.net/meme/frame.html?main=/articles/art0138.html?