The fictional world that serves as the main setting of C.S. Lewis’s Chronicles of Narnia is Narnia. It is a separate fantasy realm populated by humans, talking animals, dwarfs, giants, and other creatures.
Narnia is also the name of one country within the wider world, which includes places such as Archenland, Calormen, and the land reached during The Voyage of the Dawn Treader. This broader geography is why the term can mean both the central kingdom and the larger imagined world in casual discussion.
According to the series, Narnia is created by Aslan through song in The Magician’s Nephew. Time behaves differently there than in the human world, and people may enter through magical portals, including the famous wardrobe.
Narnia is unrelated to Middle-earth, the setting of J.R.R. Tolkien’s writings. Lewis and Tolkien were friends and members of the literary group known as the Inklings, but their fictional universes remain distinct.