Arda is the fictional world that contains Middle-earth in J.R.R. Tolkien’s mythology.
Tolkien used Arda as the name for the world created by Eru Ilúvatar within his wider mythology. Middle-earth is one major region of Arda, not the name of the entire planet or world.
Arda includes lands and seas that change across the ages. Valinor is a region of Aman, the western continent, while Beleriand is a land associated especially with the First Age. These names are often confused because Tolkien’s maps and histories describe multiple geographic levels.
The term Arda comes from Tolkien’s constructed languages and is commonly translated as “the Realm.” In later notes, Tolkien also explored cosmological ideas about Arda’s shape and its relationship to the Sun, Moon, and the wider universe.