Mundus, also called Existence, is the total sum of everything everywhere that exists within our perceivable reality. It encompasses the four universes that exist; Aetherius, heavenly paradise of the Divine Gods; Infernus, realm of Satan and the daemons; Atmah, the mortal realm created and sustained by the Divines (Includes Earth, the Milky Way, and our known universe); and finally Lenor, another mortal realm created by Lenoren which houses completely different races. There once existed a a fifth universe, known as Sargathu, created by the Dark Gods wielding the Soul Fragment of Sargeras.
Mundus is what has become known as a "completed" form of Existence - that is, it is the realization of Aetheria and Anui's goal, an ordered form of Creation (see the Creation Story). This is converse to Araboth created by Anui and Sargeras, which was not completed due to Aetheria's intervention.
Our known Existence is surrounded by the blackness of the Void, a dark ever-shifting realm of infinite darkness and chaos, where nothing remains static or orderly. This is the domain of the most ancient evil of all, the origin of all chaos and darkness, Y'Shaarj.
Our Existence (Mundus), is only one "slice" of the grander Creation, which stands as opposition to the Void.