Point of Divergence

A point of divergence is a specific event in an alternate history story that occurs differently than it did in factual history. Most alternate history authors will change a single event, creating a "ripple effect" that sets up the scenario. This divergence, which acts as the story's "unicorn in the garden" or "one big lie", may range in importance and realism from a character dropping or not dropping a piece of paper at a crucial time, to time travel and alien invasion (such as Harry Turtledove's "Balance" series, when World War II is interrupted by a fleet of lizard-people from outer space).