Bet you never expected to see a symbol resulting from the absence of symbol, did you? Guess there's a first time for everything.
Where there should be a map of Discworld in The Color of Magic, there is instead a blank space with the words "There are no maps. You can't map a sense of humor" (Appendix). Not only is it an excellent aphorism, but this phrase is also a wonderful critique on the all-too-familiar fantasy map trope.
Many fantasy novels come with maps to help the reader follow the action across the constructed landscape: Lord of the Rings, The Wizard of Earthsea, and A Game of Thrones to name just a few. In fact, it's practically a requirement these days that fantasy novels come with maps in them. Pratchett bucked the trend. In doing so, he made a statement about humor as well as the relationship between fantasy and the imagination.
When a fantasy writer creates a map of her world, she limits herself to that land created by that map. Readers expect the map to correspond to the story, and that creates a boundary on what the writer can and can't do. The borders of the fantasy map contain the writer's imagination in the same way they contain the world.
For the Discworld, the lack of map means no such constraint can be put on it. The characters can do anything and go anywhere, and it can still make sense in the context of the world. No map means no borders on the imagination constructing the Disc one book at a time.
Either that or maybe Pratchett isn't much of a cartographer. We suppose that's a possibility, too.