Roundworld
Roundworld is the Discworld term for both planet Earth and the "real" universe itself. From the Discworld's point of view it exists in a glass sphere at Unseen University, where it is cared for by Rincewind. It was created by Hex as a completely magicless zone, a magical black hole, in order to consume the huge excess of magic created after the wizards split the thaum. The vast influx of magic into a magicless space, followed by the Dean wiggling his fingers in the jar, created a new universe. The key point of Roundworld is that it doesn't contain any magic, but the wizards are fascinated by the fact it does seem to have rules of its own.
Roundworld is the focus of all three Science of Discworld novels. While it is not referred to as Roundworld yet, it makes its first appearance (so to speak) during a scene in The Colour of Magic when Rincewind and Twoflower are transported into the bodies of Earth people aboard an airplane for a brief time. However both our world and a Discworld feature in Strata.
Read more about this topic: Other Dimensions Of The Discworld