Arda is the name of the fictional world or planet where Middle-earth is located in Tolkien’s mythology.
In Tolkien’s cosmology, Arda was created within Eä, the universe brought into being through the Music of the Ainur. Middle-earth is not the whole of Arda; it is the central continental region where many of the events of The Hobbit and The Lord of the Rings occur. Other important regions include Aman in the west and the island of Númenor.
The shape and history of Arda change dramatically across the mythology. The world begins as a flat or structured realm in the early ages, while later traditions describe major transformations, including the removal of Aman from the world’s ordinary physical reach. This is why “Middle-earth” and “Arda” are not exact synonyms. Middle-earth is a region, whereas Arda is the larger world containing it.