Middle-earth is the fictional continent containing Gondor and Mordor in J.R.R. Tolkien’s legendarium.
Gondor and Mordor are regions of Middle-earth, the principal setting for The Hobbit and The Lord of the Rings. The continent also includes the Shire, Rohan, Rivendell, and many other locations connected by the stories of the Third Age.
The term Middle-earth comes from the Old English middangeard and related Germanic expressions for the inhabited world. Tolkien did not invent the basic word, but he developed it as the name for the central lands of his mythology.
Middle-earth is not the entire world of Tolkien’s mythology. Aman lies to the west across the sea, while Beleriand was a major region in the First Age that was later destroyed or changed. These distinctions explain why a location can belong to Tolkien’s world without being part of Middle-earth proper.