In C.S. Lewis’s Narnia books, the world created by Aslan’s song is Narnia.
The creation occurs in The Magician’s Nephew, the sixth Narnia book published and the first chronologically within the fictional history. Digory Kirke and Polly Plummer witness darkness give way to stars, landscape, plants, animals, and finally the speaking animals of the new world. Aslan, the great lion, sings Narnia into existence. The scene establishes the setting’s origin and explains why music, speech, and moral responsibility are important throughout the series.
Narnia is the name of both the world and, more narrowly, its central kingdom. Other places in the books include Charn, the ruined world from which the witch Jadis comes; Earth, where the human children live; and Telmar, a nation within Narnia. These names are sometimes mixed up because the series moves between worlds, countries, and historical periods. The answer is specifically Narnia, not a character, kingdom outside the world, or parallel universe.