Arda is the primary fictional world of J.R.R. Tolkien’s legendarium.
In Tolkien’s mythology, Arda is the world created by Ilúvatar for the Ainur and the Children of Ilúvatar. Middle-earth is a continent or region within Arda, not the name of the entire world. Aman, the Blessed Realm, is another major landmass in the mythology.
The history of Arda includes the creation of the world, the wars against Morgoth, the rise and fall of Númenor, and the events surrounding the Rings of Power. Tolkien revised its geography and cosmology repeatedly across his writings.
Readers often use “Middle-earth” to describe Tolkien’s setting generally, but the more precise mythological term for the world itself is Arda. The word comes from Tolkien’s invented Elvish languages and is connected with the idea of a realm.