Arda was the name of J.R.R. Tolkien’s fictional world containing Middle-earth. In Tolkien’s mythology, Arda is the world created within Eä, the larger universe of his legendarium. Middle-earth is one major region of Arda, not the name of the entire world.
Arda includes the lands of Middle-earth and Aman, the western land associated with the Valar and the Elves. Its history changes dramatically through the First, Second, and Third Ages. Cataclysmic events, including the downfall of Númenor, reshape its geography and history.
Readers sometimes use “Middle-earth” as a general label for all of Tolkien’s setting, but Tolkien’s terminology is more precise. Valinor is a region within Aman, while Beleriand is a western land of Middle-earth that is destroyed at the end of the First Age. Arda therefore identifies the broader world in which these lands exist.