The multiverse is a set of multiple distinct universes, including Earth's universe as well as Narnia's. In The Magician's Nephew, Andrew Ketterley hints at the possibility of other universes besides our own in existence. Later on, when Digory Kirke and Polly Plummer are sent to The Wood Between the Worlds via the Rings, they find that, sure enough, there are many other universes or worlds in existence, with the Wood Between the Worlds containing portal-pools to these other worlds.
Though all the universes in the multiverse are different in several ways, they all have something in common-a way to get to Aslan's Country, as Aslan himself explains in The Voyage of the Dawn Treader.
Known universes[]
Earth's universe[]
The universe containing Earth is indefinite in size and there is no known life outside of planet Earth.
Narnia's universe[]
The universe containing Narnia is magical, with sentient starts inhabiting the sky, and surrounded by Aslan's Country above, around, and possibly beneath the World of Narnia.
Charn's universe[]
The universe containing Charn is desolate and lifeless save for the queen Jadis (which she herself did via speaking the Deplorable Word). In The Magician's Nephew, Digory and Polly briefly visit the ruined Charn and inadvertently bring Jadis to Earth with them. At the end of their adventure, the world containing Charn ceases to exist and the pool in the Wood Between the Worlds that once served as a portal to that world is now dried up.
Aslan's Country[]
Aslan's Country is a divine universe where the souls of all good living creatures in the multiverse go to when they die. This universe contains the "real" versions of every world in the multiverse, including Narnia, with the Narnia that is featured in the majority of the series being a mere "testing ground" or shadow of the real Narnia in Aslan's Country. Here, everything is more real and beautiful than in the Shadowlands (as the "testing grounds" worlds are known). It is said by Aslan himself that there are ways to get to his Country in every world in the multiverse.
The Wood Between the Worlds[]
The Wood Between the Worlds is a mystical wood full everywhere with pools that serve as portals to other worlds in the multiverse. It is unknown whether it is a dimension of its own or if it is actually part of Aslan's Country.