Arda is the fictional world that contains Middle-earth in J.R.R. Tolkien’s writings.
In Tolkien’s mythology, Arda is the world created within Eä, the universe brought into being by Ilúvatar. Middle-earth is a major region of Arda, not the name of the entire world. The setting changes across Tolkien’s imagined history, including the destruction and reshaping of lands during the First and Second Ages.
Aman is another continent or realm of Arda, located to the west and associated with the Valar and many of the Eldar. Valinor is a region within Aman, while Beleriand was a western land of Middle-earth during the First Age. These names are therefore related but cannot substitute for Arda.
Tolkien’s geography was tied to his evolving creation mythology and invented languages. His map-making, linguistic notes, and narrative drafts developed over many years, which means some details changed between versions. The broad distinction remains consistent: Arda is the world, and Middle-earth is one of its principal regions.