The Discworld Companion is an encyclopaedia of the Discworld fictional universe created by Terry Pratchett and Stephen Briggs.

The book compiles a precise (and often quoted directly from the books concerned) definition of words, lives of historical people, geography of places and events that have appeared in at least one Discworld novel, map, diary, non-fiction book and the short stories "Troll Bridge", "Theatre of Cruelty", and "The Sea and Little Fishes".

The first edition was published in 1994 and listed information from all the novels up to Soul Music, as well as the first two short stories.

The second edition was published in 1997, and adds information up to Maskerade.

The third edition named The New Discworld Companion was published in 2003. It includes articles about books up to Night Watch, as well as Discworld related books and short stories. The book also contains a 10-page interview with Pratchett titled Discworld Quo Vadis?.

All of them also include information that exists on Pratchett's computer, but that he has not yet worked into a novel (for instance, William de Worde is mentioned in the first edition, six years before the publication of The Truth).

The Discworld is the fictional setting for all of Terry Pratchett's Discworld fantasy novels. It consists of a large disc (complete with edge-of-the-world drop-off and consequent waterfall) resting on the backs of four huge elephants which are in turn standing on the back of an enormous turtle, named Great A'Tuin (similar to Chukwa or Akupara from Hindu mythology) as it slowly swims through space. The Disc has been shown to be heavily influenced by magic and, while Pratchett has given it certain similarities to planet Earth, he has also created his own system of physics for it.

Pratchett first explored the idea of a disc-shaped world in the novel Strata (1981).

Exogeology

Great A'Tuin

Great A'Tuin is the Giant Star Turtle (of the fictional species Chelys galactica) who travels through the Discworld universe's space, carrying four giant elephants (named Berilia, Tubul, Great T'Phon, and Jerakeen) who in turn carry the Discworld. The narration has described A'Tuin as "the only turtle ever to feature on the Hertzsprung–Russell diagram."