The Discworld is the fictional world where English writer Sir Terry Pratchett's Discworld fantasy novels take place. It consists of an interstellar planet-sized disc, which sits on the backs of four huge elephants, themselves standing on the back of a world turtle, named Great A'Tuin, as it slowly swims through space.

The Disc is the setting for all forty-one Discworld novels; it was influenced by world religions which feature human worlds resting on turtles, as a setting to reflect situations on Earth, in a humorous way. The Discworld is peopled mostly by the three main races of men, dwarfs and trolls. As the novels progress, other lesser known races are included, such as dragons, elves, goblins and pixies.

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

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".

Great A'Tuin's sex is unknown to the inhabitants of Discworld (though in The Colour of Magic Pratchett describes the turtle as male), but the subject of much speculation by some of the Disc's finest scientific minds. The sex of the World Turtle is pivotal in proving or disproving a number of conflicting theories about the destination of Great A'Tuin's journey through the cosmos. If, as the Discworld version of the popular "Big Bang theory" states, Great A'Tuin is moving from the Birthplace to the Time of Mating, then at the point of mating the civilisations of the Disc might be crushed, simply slide off, or else the entire world will end. The hypothesis is that all stars in the sky are also worlds carried by giant turtles, and that when all the turtles meet they will mate passionately, for the first and only time; from that mating, it is hypothesised that new turtles would be born to carry a new pattern of worlds. Attempts by telepaths to learn more about Great A'Tuin's intents have not met with much success, mainly because they did not realise that its brain functions are on such a slow timescale. All they have been able to discern is that the Great A'Tuin is looking forward to something.

The other theory, described as being popular among the Discworld's academics as the Discworld version of the steady state theory, which in-universe is known as the "steady gait" theory, is that Great A'Tuin came from nowhere and is going to keep swimming through space to nowhere forever. Eric shows Great A'Tuin being made instantly from nothing, seemingly in support of the "steady gait" theory; however, the events in The Light Fantastic, in which the Great A'Tuin attended the hatching of eight baby turtles, each with four baby elephants and a tiny discworld of their own, would seem to support the Big Bang hypothesis.

The little turtles have since gone off on their own journeys. Whether this was the event the Great A'Tuin was looking forward to or merely one step towards its ultimate goal is not mentioned.

Great A'Tuin has been mentioned to frequently roll on its belly to avoid asteroid and comet collisions, or even to snatch these projectiles out of the sky which might otherwise destroy the Disc. These stunts do not affect the Disc's population, other than to induce severe seasickness on anyone who happens to be looking at the night sky at that time. A'Tuin has been known to do more complex rolls and corkscrews, but these are rarer. This is similar to real-world sea turtles' habit of rolling over with their shell down to protect themselves from sharks.