Arda is the name of the world in J.R.R. Tolkien’s mythology that includes Middle-earth.
In Tolkien’s creation story, Eru Ilúvatar brings the universe, Eä, into existence, while Arda is the world within that universe. Middle-earth is not the whole of Arda; it is one major region or continent of the world, especially important in the histories of the First, Second, and Third Ages.
Arda’s shape and geography change across Tolkien’s mythology. The destruction of Númenor and the reshaping of the world mark major transitions, and the Undying Lands of Aman are separated from the mortal realms by the end of the Second Age. These distinctions can be difficult because Tolkien developed the mythology across many drafts and posthumously published texts.
Arda is therefore more precise than Middle-earth when referring to the entire world of Tolkien’s legendarium. Eä means the created universe, while Aman is a separate western land within Arda.