The term "multiverse" (essentially multiple universes) refers to the collection of all individual dimensions (or planes) of existence, including both Heaven and Hell dimensions. For an unknown reason, in the nefarious plans of many villainous Higher Beings and Deities the dimension known as Earth has played a central role.
The multiverse is sometimes incorrectly referred to as universe, a term which actually denotes only a single dimension.
Behind the scenes
Fans have coined the term "Buffyverse" to refer to the shared fictional universe in which Buffy the Vampire Slayer and Angel are set. The term "Angelverse" is sometimes used to refer to the setting specifically depicted in Angel as a self-contained fictional universe, distinct from the one depicted in Buffy the Vampire Slayer. Alternative terms that are sometimes used by fans include "Bangelverse" or "Slayerverse".
It is worth noting, that these terms are (for obvious reasons) not used by any of the characters to refer to the concept of the entire collection of dimensions. The term "multiverse" itself was used only once in Lorne: The Music of the Spheres.