Most of J.R.R. Tolkien’s major stories are set in Middle-earth.
Middle-earth is the inhabited part of Arda, Tolkien’s imagined world, rather than the name of the entire universe. It is the primary setting of The Hobbit and The Lord of the Rings, and it also appears throughout the histories later collected in The Silmarillion.
The name comes from the Old English term middangeard and related Germanic expressions for the human world. Tolkien’s use is therefore connected to historical language, not to a modern scientific idea about a planet’s interior. Middle-earth includes regions such as the Shire, Rohan, Gondor, Mordor, and Mirkwood.
A common mix-up is treating Middle-earth and Arda as synonyms. Arda is the world, while Middle-earth is one major region within it. Tolkien’s fictional geography changes across different ages, so not every place or historical event belongs to the same period.