Universe

A Universe (also sometimes improperly called dimension) is the group of all matter that exists within a single phisical space. By phisical travel it is only possible to visit the contents of the universe you started in, no matter how far you get, because other universes exist beyond physical space. To travel to other universes is necessar to travel in an other hipotetical dimension, causing many people to call another dimension what is actually another universe only on another coordinate on the other dimension. The same as calling another position on a line, another lenght.

It is unclear if energy can travel between universes or it is confined to a single universe the same way matter is.

To this day the Real World is the only known non-fictional universe, but there are many theories if it is or not the only existing universe. Many believe that there are infinite universes including every possible reality. It is also possible, and believed by many, the existance of one or more universes where the soul of dead rest after dying.

Note that while many universes are called Earth-[code], the universe is not confined on the single Planet Earth, but also every other celestial body and also the outer space in that reality. They are identified by the name "Earth" just because the most notable differences between them are on planet Earth.

Fictional universes are all universes in wich a fictional story takes place. Every time a story is written or even just tought, a universe is created (or discovered if you believed every universe always existed). It is possible that a single story takes place in more than one single universe, and this is made possible by giving the people of one fictional universe the ability to travel between universes.

It may also happen that a crossover revails that two or more fictional stories happened in the same universe. Conventionally, if a crossover never happened, all stories are considered to take place in different universes (for example, even if they both take place in realistic worlds, there's no reason to consider the movie Ghostbusters taking place in the same universe as the movie series Indiana Jones).

A group of Universes is called Multiverse.