From Discworld MUD Wiki
The Roundworld means the earth, or refers to out of game references.
In the context of time it represents out of game time units, as opposed to in game time, which is 10/3 faster.
In the Discworld novels, the wizards of Unseen University created the Roundworld universe to dispose of excess energy released by splitting the thaum.
- See also the lspace wiki article.