"Can you imagine nothing? No up, no down, no black, no white, no life at all."
- The Doctor
The Void is the nothingness that exists between the billions of universes that are all stacked on top of each other. The Time Lords call it "The Void",while the Eternals call it "The Howling", and others call it "Hell".
The Void was only accessible by traveling in a Void Ship.