Alternate History is the most frequently used term for the fictional genre which often presupposes a change of a minor historical event that producing an incredible series of changes in the world, diverging it from Our Timeline (OTL) and creating a new, alternate and parallel world.
In the scientific field, these Alternate Timelines and parallel worlds were theorized by physicists in the late-20th Century. They suggested that time has "branches" as a result of numerous points of divergence (POD) in the past. While some believe many branches occur in our universe, due to numerous departure points, others speculate further that all branches occur simultaneously, and perhaps in other universes. The changes would represent "alternate histories," differing in varying degrees from history as it unfolded in OTL.
This scientific knowledge has become inspiration to the literary world, and especially on the Internet, where Alternate Timelines began appearing in the mid- to late-1990s first on newsgroups, then on individual Webpages and most recently on Althistory Wikipedia.
In literature and in other media, especially film and television, Alternate History has often been used in conjunction with, and are often caused by, time travel. In the film series Back to the Future, a lead character, "Doc" Brown, explained the concepts of Alternate History, Alternate Timelines and points of departure, as well as time travel.
Alternate History have also been used as a literary device by historians to explore how social and political changes in the past may have affected events in OTL.
This is a brief summary of the Alternate History genre. For a more detailed description of Alternate History as a fictional genre, please visit Wikipedia.org