From The World and Everything In It
Strictly speaking, an alternate history is a novel readjustment of the primary world's history in order to answer a question called a "what if". It becomes a scenario that not only answers the question but also fleshes out the initial premise. For example, "what if Julius Caesar had died of grump before dividing all Gaul in three parts?" Some answers are obvious, like we'd never ever have to worry about the Ides of March. But alt-historians are interested in the domino effects these kinds of changes, or points of departure have on future history. In some respects, it's an interesting game to consider what sort of immediate consequences such a change in the timeline might engender.
To expand the example, let us consider: "Mark Antony, a Roman politician and general, was an important supporter and the loyal friend of Gaius Julius Caesar (the same who in our what-if had died of gump some years previously) as a military commander and administrator, being Caesar's second cousin, once removed, by his mother Julia Antonia. After Caesar's assassination, Antony formed an official political alliance with Octavian (Augustus) and Marcus Aemilius Lepidus, known to historians today as the Second Triumvirate. The triumvirate broke up in 33 BC. Disagreement between Octavian and Antony erupted into civil war, the Final War of the Roman Republic, in 31 BC. Antony was defeated by Octavian at the naval Battle of Actium, and in a brief land battle at Alexandria. He and his lover Cleopatra committed suicide shortly thereafter."
We can immediately see that Caesar would never have been assassinated, because he is already dead, there would have been no triumvirate, no disagreement between Octavian (later Emperor Augustus) and Mark Antony, no civil war, no Battle of Actium, no Cleopatra episode and possibly no Emperor Augustus at all. That's a major chunk of Roman history that will need to be rewritten entirely, all because one man died before he did in another timeline. The entire future course of the world's history is changed by one chance death.
The World is not a strict alternate history because it is not point of departure driven. There is no one single moment in history that changes, thus giving rise to a bifurcation in the timeline of the primary world. It is a world with its own history from start to finish. Likewise, a strict alt-history takes place on Earth, the same physical space we exist in. Of course, as the courses of history change, some things may be altered. Perhaps a dam won't be built on the Nile, for example. But the constraints of the game generally allow for Nature to take her course identical to what happened in the primary world. Thus, a major earthquake strikes the New Madrid fault in the 1830s whether it's American settlers, Norse Vikings or blue skinned space aliens from Aris in the story line.
Also, The World does not take place within the same physical space as Earth. Gea is a different planet orbiting a different Sun within a different (but similar) solar system.
What the World does, however, is parallel some aspects of primary world history. So, they do have an emperor Agustas, there are Jews and Christians and emperors of various sorts. However, there are also things that never happened in history and semi-legendary people and events that are historical. Entirely mythological places and peoples have come into being that have only literary echoes *here* in the primary world or else no connexion at all.