The fictional land visited through a wardrobe in C.S. Lewis's novel is Narnia.
Lucy Pevensie discovers Narnia by stepping through a wardrobe in The Lion, the Witch and the Wardrobe. The wardrobe serves as a doorway from wartime England into a parallel world populated by talking animals, dwarfs, fauns, giants, and other beings.
Narnia is also the name of the country at the center of the larger setting. Lewis's seven books include neighboring lands and regions such as Archenland, Calormen, Telmar, and the land beyond the sea. The stories span a fictional history from Narnia's creation to its final destruction.
A frequent mix-up is treating Narnia as only the wardrobe or only the first book. The wardrobe is the portal, while Narnia is the world and kingdom that the Pevensie children and other protagonists enter. Lewis published the first Narnia novel in 1950 and completed the series in 1956.