http://en.wikipedia.org/wiki/Alternate_history
Quote:
Definition
In writing an alternate history, the author makes the conscious choice to change something in our past. According to Steven H Silver, alternate history requires three things: 1) the story must have a point of divergence from the history of our world prior to the time at which the author is writing, 2) a change that would alter history as it is known, and 3) an examination of the ramifications of that change.[4]
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Quite a bit of good information at that link explaining the different types of AH. As well as what is and isn't AH.