Arda is the fictional world containing Middle-earth, Valinor, and Beleriand in J.R.R. Tolkien’s mythology.
Arda is the realm shaped by the Valar under the authority of Eru Ilúvatar. It includes the lands and seas where the major events of Tolkien’s legendarium occur. Middle-earth is one part of Arda rather than a name for the entire created universe.
Valinor lies in Aman, the western region associated with the Valar and the Eldar. Beleriand is a western land of the First Age, later destroyed and submerged during the War of Wrath. Middle-earth remains the principal setting of The Hobbit and The Lord of the Rings.
The larger universe is called Eä, the created universe brought into being through the Music of the Ainur. This distinction causes frequent confusion: Arda is the world or realm, while Eä is the wider cosmos. Tolkien’s terminology changed in detail across drafts, but this broad hierarchy is central to the published mythology.