A Universe, when dealing with fiction, is simply put the world in which the characters and events of a particular franchise exist - they can exist as singular realities or have several connected realities, universes that have many alternate realities are often labelled Multiverses.

Normally a universe exists independently from all others in the Omniverse but can interact with outside universe in special events known as "crossovers" - these can vary from a simple cameo by one character in a series outwith their own or a large-scale company wide crossover involving hundreds of characters and settings: on rare occassions a crossover can create an entirely new universe or result in elements of one universe merging with another.

Every universe has its own unique laws of physics (or lack of) and time-line as well as characters and events which when merged together make up the general feel of a franchise - some universes are almost identical to our own while others are wildly different, all universes - no matter how distant - are a part of the Omniverse.

See also