The word Zith has carried down through thousands of generations of mortals from the original True Race. Namely to a particular group of their many descendants known as the Eneladrin. It means a mixture of reality, desire, pursuit of unattainable consequences, and countless more abstract truths. It means strife bound by equality and balance in an eternal war.
As an example: the forces of good conquer a force of evil; but in doing so the force of good must sacrifice an equally important part of itself to the force of evil. The center, the material plane, the Zithket is the battleground for many exterior ideologies and their armies.
It is from this very vague, philosophical, and abstract introduction that the doorway into this world is revealed.