Arda is the fictional world in which J.R.R. Tolkien’s Middle-earth stories take place.
Arda is the world created by Eru Ilúvatar in Tolkien’s mythology. It is part of the larger universe called Eä, which came into existence through the Music of the Ainur. Middle-earth is not synonymous with Arda: it is the central region of the world, while Arda includes other lands and seas.
The history of Arda is divided into major ages. The First Age includes the wars of Beleriand and the stories later collected in The Silmarillion. The Second Age includes the rise and fall of Númenor and the forging of the Rings of Power. The Third Age contains The Hobbit and The Lord of the Rings.
Valinor is a realm in Aman, not the name of the whole world. Eä is broader still, describing the created universe. These layered terms are why Arda is the specific answer when the question asks for Tolkien’s world.