Prime Universe

The Prime Universe is the universe from wich spawned all other universes within a single multiverse and in most cases it is the prototypical incarnation of the story.

Not every multiverse necessarly has a Prime universe, because not all universes in it has to spawn from one single story. For example if you consider a Disney Multiverse, there are universes that spawned from another one in it, but many of them spawned from other stories outside the multiverse (for istance most classic movies spawned from the Grimm's stories).