Arda is the fictional world where Middle-earth is located in Tolkien’s legendarium.
In Tolkien’s cosmology, Arda is the world created within Eä, the universe of his mythology. Middle-earth is not the entire world; it is one major region of Arda, especially prominent in the stories of The Hobbit and The Lord of the Rings.
Arda includes lands and seas that change dramatically across the ages. Aman, the realm west of the sea, is associated with the Valar and the Elves, while Beleriand was a major region in the First Age before it was destroyed or submerged during the War of Wrath.
Readers sometimes use “Middle-earth” as if it means Tolkien’s whole universe. The term is more geographically limited. Arda also should not be confused with Valinor, which is a city or region within Aman rather than the name of the entire world.