Point of divergence
In discussion of counterfactual history, a point of divergence (POD), also referred to as a departure point or divergence point (DP), is a historical event with two possible postulated outcomes. Typically these represent the actual course of historical events (Napoleon lost the Battle of Waterloo) and another, such as Napoleon won the Battle of Waterloo.
In alternate history fiction, the POD is used as the starting point for the extrapolation, as it is indeed in much of the science fiction genre.
In Philip K. Dick's The Man in the High Castle, an alternate history novel in which Germany and Japan win World War II, the point of divergence is Franklin D. Roosevelt's attempted assassination by Giuseppe Zangara in 1933, which did take place in its timeline and led to an Axis victory in a prolonged Second World War in 1948. An Axis victory in World War II is probably the most common use of this plot device in contemporary alternate-history SF.
In Ward Moore's Bring the Jubilee, the point of divergence is an alternate-history victory of the Confederates at the Battle of Gettysburg in 1863, which means that in 1865, the Confederate States of America wins the American Civil War.
In Eric Flint's alternate history (a time travel variation) he postulates an Assiti Shard event which juxtaposes parts of our planet including the town of Grantville, West Virginia in both space and time—a twist on scientist's referring to a space-time continuum in relativistic (Einsteinian) physics. Similar implausible points of divergence are often referred to as being done by alien space bats.
One multiverse theory posits that PODs are occurring all the time, with an infinite variety of possible outcomes that each creates a universe, this having been used as a premise to the 1990s US television series Sliders. Speculative fiction is full of universes based loosely on the concept of multiple universes, including many fantasy milieus. An example from fantasy is the Discworld series which jumps between several distinct settings.
A video game that shows signs of divergence points is the Japanese visual novel game, Steins;Gate, in which the main character travels between world lines, but ultimately finds out that his actions lead to a Dystopian world and had to travel back in time to fix what had happened.
In the 2003 anime series of Fullmetal Alchemist, there exists a gateway that can be conjured by alchemists that acts as a source of all knowledge and energy; towards the end of the series, it is revealed that this gateway connects the world of the anime with the real world, set during the first decades of the 20th century. It is revealed that the two worlds shared a common history until their histories diverged, apparently due to the success of alchemy in one world and that of modern physics in the other.