Facet

A Facet is a universe or world within the Diamond of Existence.

Each Facet represents a different version of the space that entities can inhabit, say.

Things that could separate otherwise identical Facets could include rules system, Chief Ruler, laws of physics, the vagaries of probability, inhabitants, influencing events from other universes...

Some Facets are roughly equivalent to a planet, i.e., a "world." Others are galaxies, universes, or even multiverses, all contained in the meta-multiverse.

Sometimes it is possible to travel to other Facets via mundane/practical means such as interstellar travel or Spelljamming. Other times interplanar teleportation, gating, or the intervention of divine power is necessary.

Many Facets are so isolated that the inhabitants could never prove theirs is not the only Facet - and so, effectively, it is the only Facet. Other Facets are simply unaware of the Diamond.

The word "Facet" is roughly equivalent to "campaign world".

A list of known campaign worlds can be found at http://diamondpantheon.com/wiki/w/index.php/Category:Campaigns