The Wood between the Worlds is a pond-filled forest in The Magician's Nephew (1955), the sixth book in The Chronicles of Narnia by C. S. Lewis. Each pond is a portal that provides instant transportation to a "world" such as ours and Narnia's.
The Wood between the Worlds is so named by Polly Plummer, who is transported there when Digory Kirke's Uncle Andrew tricks her into picking up a magic yellow ring. She soon falls asleep, and when Digory arrives later the children are both disoriented; they aren't sure how long they have been there or even who they are. The state of lassitude is explained by the Wood being a place where nothing ever happens, unlike the different worlds that it connects. Later, Empress Jadis of Charn is brought to the Wood and appears to fall ill because of it, weakening so much that the children are stronger than she. Jadis soon recovers after leaving the Wood, but she never mentions it subsequently. Digory speculates that her mind is unable to hold memories of the Wood.
The Wood is described as having a warm temperature and containing a large number of leafy trees with canopy foliage so thick that the sky (assuming there is one) cannot be seen from the ground. Despite the thick foliage, a strong light does penetrate to the woodland floor, clearly illuminating objects. The salient feature of the wood, other than the trees, is the presence of many pools of water. Initially, the pools appear to be just shallow puddles. However, when someone jumps into one of the pools while wearing another magic ring, the pool of water transports the wearer to a different world. When a world is destroyed by having all life removed (as happened to Charn in The Magician's Nephew), the pool dries up.
The wood is implied to be a place linking all worlds, including Narnia, Charn, and Earth. A sense of slothfulness grips all visitors, including Jadis. However, it affects Jadis negatively: she shrieks in despair that the wood is "killing" her, and she is sickly and pale; when Polly and Digory arrive in the wood, their experience is pleasant and relaxing. It could be considered a multiverse of sorts, existing outside the normal physical realities. In Paul F. Ford's Pocket Companion to Narnia, a diagram shows the Wood Between the Worlds to be located in Aslan's Country, sitting on top of all worlds.