The fantasy world in C.S. Lewis’s The Chronicles of Narnia is called Narnia. The name refers both to the wider fictional world and, in some contexts, to the land ruled by Aslan and visited by human children.
Lewis’s seven-book series combines talking animals, mythical creatures, children from the real world, and Christian imagery. The books were published between 1950 and 1956, although the internal chronology of Narnia differs from publication order.
Narnia is not the same setting as J.R.R. Tolkien’s Middle-earth, Ursula K. Le Guin’s Earthsea, or George R.R. Martin’s Westeros. Those are separate fictional worlds created by different authors.
The series begins with The Lion, the Witch and the Wardrobe in publication order. The wardrobe provides the best-known doorway between the ordinary world and Narnia, although later books use other routes between worlds.