Arda is the name of J.R.R. Tolkien’s fictional world that includes Middle-earth and Aman.
In Tolkien’s mythology, Arda is the realm or world created within Eä, the wider universe. It contains the lands where the major histories of the legendarium unfold, including Middle-earth and the western land of Aman. The name is especially important because it refers to the whole world, not merely to the continent most commonly called Middle-earth.
Tolkien’s cosmology changes over time. In the earliest conception, Arda is associated with a flat world containing lands and seas in a different arrangement. Later writings describe the reshaping of the world and the removal of Aman from ordinary access after the downfall of Númenor.
Arda is therefore often confused with Eä, which means the created universe, or with Middle-earth, which is only one major region of Arda. Valinor is also narrower: it is a land within Aman, not the name of the entire world.