Arda is the name of J.R.R. Tolkien’s world that includes Middle-earth and Valinor.
In Tolkien’s mythology, Arda is the realm made for the Children of Ilúvatar, especially Elves and Men. It is part of the larger universe called Eä, which was brought into existence through the Music of the Ainur. Valinor is a region of Aman, while Middle-earth is another major part of Arda.
Readers often use “Middle-earth” as if it means the whole setting, but Tolkien’s geography is broader. Middle-earth is a continent or region within Arda, not the name of the entire created world. Arda also changes over time, including the reshaping associated with the destruction of Númenor and the transition from a flat world to a round one.