A Universe is a reality in which events take place. We live in what is called on Infinite Histories Real Life 00.
On Infinite Histories, we have many universes in our database, and many are at different time periods. Some are still in the primeval era, while some may be many years into the future, in respect to Real Life.
Some universes may be divergent realities of Real Life, where a single event is changed to create a new timeline. An example of this is the Romanum universes, where the Roman Empire never fell.
Others may be embellished. Embellished histories are not quite as historically (or even scientifically) vigorous as Divergent histories, but still not original enough to count as Fantastical histories. Embellished histories are unlikely to have happened in real life and usually require multiple or widescale alterations to real history to make them work. Less attention is put into historical accuracy in Embellished histories and more is left to the imagination of the writer.
As stated above, the final type of alternate universe is the Fantastical history. Fantastical histories are completely original; history can go in completely unexpected directions and magic or new laws of science are the norm. Fantastical histories are impossible in real life and require complete alterations to the history of the Earth and the nature of the universe which cannot be achieved by any event. No attention need be put into historical accuracy in Fantastical histories and the world is the writer's play-thing.
Universes are one of the main focuses of Infinite Histories. It is hoped that in the future many alternate realities will be in existence, and that users will let their 'what-if' questions run wild!