Narnia is the land created by Aslan in C.S. Lewis's The Magician's Nephew.
Published in 1955, The Magician's Nephew is the sixth Narnia book by publication order but is set at the beginning of Narnia's fictional history. The children Digory Kirke and Polly Plummer witness the creation of the world after traveling between different places through magical rings and pools.
Aslan sings Narnia into existence, bringing stars, plants, animals, and other features into the new world. The scene explains why Narnia contains talking animals and establishes Aslan's role as its creator and king.
The novel also introduces several details that connect to The Lion, the Witch and the Wardrobe. Digory becomes the professor whose wardrobe later opens the way to Narnia, while the evil Jadis survives the destruction of Charn and eventually becomes the White Witch. Charn and Narnia are therefore different worlds, not alternate names for the same land.