Arda is the world or realm containing Middle-earth in J.R.R. Tolkien’s mythology.
Tolkien’s wider cosmology distinguishes several related terms. Eä is the created universe, while Arda is the world within it. Middle-earth is not the entire world; it is one major region of Arda, especially the lands central to The Hobbit and The Lord of the Rings.
Aman is the western continent associated with the Valar and the Elves, while Valinor is the realm of the Valar within Aman. These names are often treated as interchangeable in casual discussion, but Tolkien’s geography gives them different meanings.
Arda changes across the mythology. Its early history includes the shaping of the world by the Valar, the wars of the First Age, the destruction of Beleriand, and the changing form of the seas and lands. This layered cosmology is one reason Tolkien’s fictional setting feels more like a complete mythology than a simple map for one novel.