Alternate histories (synonyms: chronicles, archives, annals, timelines) are fictional worlds or fictional histories where a single event or series of events has had a different outcome compared to our own real world. The losing side of a war may have won in this version of history, a character may have turned left instead of right, or a common innovation (combustion engines, telephones, gunpowder, etc.) may never have been invented. Worldbuilders creating alternate histories imagine such a world and how it would develop under those new circumstances. A fictional world may center entirely on an alternate history, or alternate histories may exist as alternate timelines or alternate dimensions in worlds with those elements.
Worldbuilders may find it useful to create a timeline for their alternate history.
Alternate histories are often combined with either science fiction or fantasy elements. Worldbuilders may imagine how history would have unfolded if aliens made contact with our world in the early 19th century, or if portals to a magical world began appearing in the 1960s, or how the presence of superheroes may have drastically changed the outcome of World War I.
Worldbuilders creating alternate histories generally begin in one of two places: they may have a desired outcome in mind (air travel is invented 100 years earlier, the Vikings fully and permanently colonize North America beginning in the late 10th century) and consider all the prior events that must lead to that outcome, or they may make a change in our own timeline (Edison never existed, the Enola Gay was shot down) and imagine how history would have changed due to that change. In practice, worldbuilders often mix these two approaches.
There may be a single event that changes the course of history, or a series of events scattered across time and space. These events may be major ones, or could be minor changes that seem inconsequential but actually result in drastic divergences. For some worldbuilders, defining the event or events that create an alternate history is important. Others may wish to leave these details soft and undefined, wishing instead to focus on the ramifications rather than the impetus.
Creating an alternate history requires extensive research of the time period in which that history diverges from our own.