Arda is the fictional world that includes Middle-earth in J.R.R. Tolkien’s mythology.
In Tolkien’s legendarium, Arda is the world created within the larger cosmological setting of Eä. Middle-earth is not the whole planet or world; it is one major region of Arda. Other important regions include Aman, the western land associated with the Valar and the Elves, and Beleriand, a First Age region later destroyed or submerged.
Arda changes dramatically across Tolkien’s ages. The wars of the First Age reshape the western lands, while the downfall of Númenor causes a major transformation in the world’s geography. By the time of The Lord of the Rings, the stories focus mainly on Middle-earth and its peoples.
A common mix-up is using Valinor as the name for the entire world. Valinor is a region within Aman, and Aman is itself part of Arda. “Middle-earth” is also a geographical term, not the complete name of Tolkien’s cosmic setting.