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", however, the point of divergence 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 Two is interrupted by a fleet of lizard-people from outer space).