The fictional world where C.S. Lewis set his seven Chronicles of Narnia books is Narnia. The name refers both to the central land and, in broader usage, to the imaginative world of the series.
Narnia contains talking animals, mythical creatures, human visitors, and a long history shaped by the lion Aslan. The books move across different eras, so characters from Earth may enter Narnia at widely separated points in its chronology.
Lewis published the seven books between 1950 and 1956. The series begins with The Lion, the Witch and the Wardrobe in publication order and ends with The Last Battle. The Magician’s Nephew, although chronologically earlier in the fictional history, appeared as the sixth published book.
Middle-earth belongs to J.R.R. Tolkien, Earthsea to Ursula K. Le Guin, and Prydain to Lloyd Alexander. Those are separate fantasy settings, not alternatives for Lewis’s world.