Arda is the name of the fictional world containing J.R.R. Tolkien’s Middle-earth stories.
In Tolkien’s mythology, Arda is the world made by the divine beings known as the Valar within the larger cosmological creation of Eä. Middle-earth is not the entire world: it is one major region of Arda, especially important during the First, Second, and Third Ages.
Other places often mistaken for the world’s name are Valinor, Aman, and Beleriand. Valinor is a land in Aman, while Aman is a continent west of Middle-earth. Beleriand is a region that existed mainly during the First Age before much of it was destroyed and submerged.
Tolkien’s maps and notes changed as his legendarium developed, so geographic details do not always align perfectly across every text. The broad distinction remains useful: Arda is the world, while Middle-earth is its central inhabited region in the best-known narratives.