The fictional world commonly associated with J.R.R. Tolkien’s novels is called Middle-earth.
Middle-earth is the setting for The Hobbit, The Lord of the Rings, and much of Tolkien’s legendarium. It is not the entire planet of Arda; rather, it is a major region of that world during the periods described in Tolkien’s stories. Places such as the Shire, Gondor, Mordor, and Rohan are all located in Middle-earth.
The term comes from the Old English middangeard and related Germanic ideas of the inhabited world. Tolkien adapted the concept for his mythology rather than inventing the phrase from nothing. Readers sometimes use “Middle-earth” as a synonym for all of Tolkien’s fictional universe, but the more precise distinction is that Arda is the world, while Middle-earth is one of its principal regions.