A Multiverse is a concept common in many fictional settings that embraces the concept of many connected Universes, which are all owned by the same company or in some rare cases several companies, a Multiverse is not infinite like the Omniverse but are vast in scale and can often grow in time as companies expand their characters and worlds.

Multiverses can also be erased and replaced with a standard Universe, often as a result of a company rewriting events and characters (known as retcon) - however most Multiverses revert back to their previous state soon afterwards so permanent conversion from one concept to the other is rare.

A Multiverse includes all alternate universes and time-lines within a specific franchise but doesn't include those outwith it - for example the Marvel Multiverse comprises of everything you can imagine to do with Marvel and its heroes but it has nothing to do with the DC Universe (which changes between a Multiverse and Universe concept fairly frequently).