J.R.R. Tolkien gives the name Arda to the world or inhabited realm in which most of his Middle-earth stories occur. Arda includes Middle-earth as well as other lands and seas shaped by the history of Tolkien’s legendarium.
In Tolkien’s cosmology, Arda is part of Eä, the created universe. Valinor is a region in Aman, a continent of Arda, while Middle-earth is another major region. These names describe different levels or places and therefore are not interchangeable.
Arda changes dramatically across the ages. The destruction of Beleriand at the end of the First Age and the changing shape of the world after the downfall of Númenor are major examples. Tolkien’s mythology also presents Arda as a world marred by the actions of Melkor.