Dimensional tear through the fabric of reality and can serve as a bridge between many worlds. Exists on a extraplanar "immaterial" plane of existence as opposed to the physical "material" plane of existence.

It exists yet does not exist at the same time.

However sometimes there is discrepancies between "The Void" and "The Interdimensional Rift".

And some characters in FF are able to traverse or influence the phenomenon. Primarily GILGAMESH, Shinryu, and Omega.

And there are of course others with an affinity for the Void. Such as Enuo and his successor ExDeath.