Multiverse

A Multiverse is a group of two or more Universes. It can be identified by similarities between its hinabitants (for example the TMNT Multiverse is the group of all universes inhabited by ninja turtles), or by all universes phisically linked to each other (so Dimension X can be part of the TMNT Multiverse, even if its not inhabited by the Turtles, because they are able to visit it).

Anyway most times the Multiverse is identified by the company that created it (for example the Marvel Multiverse can be identified as the group of universes created by comics company Marvel), even if they are not linked to each other.