Most of J.R.R. Tolkien’s stories are set in the fictional region called Middle-earth.
Middle-earth is the central landmass of Tolkien’s imagined world during the periods covered by The Hobbit and The Lord of the Rings. It includes locations such as the Shire, Rivendell, Gondor, Mordor, Rohan, and Mirkwood. The term is Tolkien’s modern English rendering of a concept related to the Old English middangeard.
Middle-earth is not the entire planet. Tolkien’s world is called Arda, and Middle-earth is one major part of it. Other important regions include Aman, the western land containing Valinor, and the sunken island kingdom of Númenor.
The phrase is often used casually to mean Tolkien’s whole mythology, but that usage is broader than its precise geographical meaning. The stories span different ages, and the shape and history of the lands change dramatically over time.