The general setting of all alternate histories. A universe that is home to infinite possible alternate timelines.

Also, in the context of AH.com The Series, AH.com The Next Generation and AH.com Enterprise, the Multiverse serves as the main setting of those shows.

The term was coined by a certain William James. Yes, the pragmatist philosopher from the US of A.

AH.com The Series

AH.com The Next Generation

AH.com Enterprise