Arda is the fictional world containing Tolkien’s Middle-earth, Valinor, and Númenor.
In Tolkien’s legendarium, Arda is the world created by Ilúvatar and shaped by the Valar. Middle-earth is one major region of Arda, while Valinor lies in Aman, a blessed western land. Númenor was an island kingdom in the Great Sea before its destruction.
The word is sometimes used loosely to mean Middle-earth, but Tolkien’s geography distinguishes them. Middle-earth is a continent or region within the wider world, not the name of the entire cosmic setting. Eä, meanwhile, refers to the created universe that contains Arda.
Arda changes across the legendarium’s history. The destruction of Númenor and the reshaping of the world help explain why later maps and stories differ from the landscapes described in The Silmarillion and The Lord of the Rings.