In alternate history fiction, the Point of Divergence, or POD, is the moment when the fictional world stops following the same course as our timeline.
Many alternate history stories use a battle as a Point of Divergence. While this can involve changing which side won, the "butterfly effect" means that a significant change can be much smaller. For example, the POD for the Trail of Glory timeline is that Sam Houston only took a minor leg wound at the Battle of Horseshoe Bend, instead of being hit in the groin.
Some alternate histories, such as those in the Assiti universe, involve time travel or time displacement. In these stories, the POD is set by the arrival of the traveler(s), or the displaced person(s) or area(s). For example, the POD for the 1632 timeline is the arrival of Grantville in late May of 1631.