Arda is the name of the world in which Middle-earth exists in J.R.R. Tolkien’s mythology.
In Tolkien’s cosmology, Arda is the created world, originally shaped by the Valar under the authority of Ilúvatar. Middle-earth is one major region of Arda, especially important during the First, Second, and Third Ages. Other regions include Aman, the Blessed Realm, which contains Valinor.
The term is often confused with Eä, which refers to the entire created universe, or with Valinor, which is a land within Aman. Beleriand is another region, largely destroyed or transformed by the end of the First Age. These names describe different scales of Tolkien’s geography rather than interchangeable names for one place.
Arda’s shape and geography change through the mythology. The destruction of Númenor alters the world, and later traditions describe the removal of Aman from the circles of the world. This layered history is one reason Tolkien’s setting resembles a mythology rather than a single static map.