The fictional world visited by the Pevensie children in C.S. Lewis's chronicles is Narnia.
Narnia is the principal setting of The Chronicles of Narnia, a series of seven fantasy novels published between 1950 and 1956. The Pevensie children enter it through different routes, including a wardrobe, a railway platform, and a magical painting. In Narnia, they encounter talking animals, mythical creatures, kings and queens, and the lion Aslan.
Narnia is also the name of the world in which the kingdom of Narnia exists, although the books sometimes use the word more narrowly for the central country. Other lands in the wider world include Archenland, Calormen, and the land of the giants.
Narnia is often confused with Middle-earth, the setting of J.R.R. Tolkien's writings. Lewis and Tolkien were friends and fellow members of the Inklings, but Narnia and Middle-earth are separate fictional universes created by different authors.