Arda is the fictional world that contains Middle-earth in J.R.R. Tolkien’s legendarium.
In Tolkien’s cosmology, Arda is the world created within Eä, the larger universe brought into being by Ilúvatar. Middle-earth is one region of Arda, especially the setting for The Hobbit and The Lord of the Rings. The name does not mean a separate planet in the modern scientific sense; it refers to the inhabited world of Tolkien’s mythology.
Arda changes dramatically across Tolkien’s imagined history. It includes the lands of the First Age, such as Beleriand, and later regions including Middle-earth, Númenor, and Valinor. Cataclysmic events alter its geography, most notably the destruction and reshaping associated with the end of the First and Second Ages.
Readers often use “Middle-earth” as if it names Tolkien’s entire universe. More precisely, Middle-earth is a major part of Arda, while Valinor is a blessed land in Aman and Númenor is an island kingdom of the Second Age.