Arda is Tolkien’s invented name for the world containing Middle-earth and Valinor.
In Tolkien’s mythology, Arda is the Earth or world that the Valar shape within Eä, the larger universe created by Eru Ilúvatar. Middle-earth is one major region of Arda, while Aman, the land of the Valar, lies to the west. Valinor is a realm within Aman rather than a synonym for the whole world.
The meaning and history of Arda develop across Tolkien’s writings. In the earliest stages of the mythology, the world’s geography and cosmic structure differed in important ways from the versions later presented in The Silmarillion and related texts. The term nevertheless provides a useful label for the complete world of the legendarium.
Readers often confuse Arda with Middle-earth because Middle-earth is the setting of Tolkien’s best-known narratives. Middle-earth is only a continent or region of Arda, not the entire created world. Eä is broader still, referring to the universe.