Arda is the fictional world containing Middle-earth in J.R.R. Tolkien’s mythology.
Arda is the name Tolkien gave to the world created within his larger mythology. Middle-earth is one major region of Arda, not the name of the entire planet or universe. The stories of The Hobbit and The Lord of the Rings take place mainly in Middle-earth during a particular period of its history.
Arda’s history begins with its shaping by the Valar, powerful beings who serve the creator Eru Ilúvatar. Tolkien’s writings describe major changes to the world across the First, Second, and Third Ages, including the destruction and reshaping of lands.
A frequent misconception is that “Middle-earth” means the whole setting. Tolkien derived the term from an older expression for the inhabited world, while Arda functions as the broader world-name in the mythology. Valinor, Númenor, and Middle-earth are all associated with Arda at different points in its history.