A demiplane hidden inside the Ethereal Plane, where the minds of mortal sleepers visit while they dream. It is a vast eternity of constantly-changing dreamscapes. It is very difficult to reach it by magic; the only way Dora knew of was through obscure occult rituals rather than actual spells. She thought of it as a place haunted by Night Hags who stride through the land of dreams like demigods, with connections and windows to every other plane of existence. Anything that can dream can touch it, slip in and out, and contribute to its ever-shifting madness.
In the Act I Epilogue, the Prisoner attempted to contact the party by searching for their memories via their subconscious connection to the Dimension of Dreams.
In the Act II Interlude, it was revealed as a place where the Mad Poet's book had been hidden for a time, before it was carried into Golarion.
In Chapter 62, it was revealed as the plane inside of which the Plateau of Leng dwells.
In Chapter 73, Ray’s ghost experienced a partial memory of walking down Market Street with Daria. While dwelling on the Ethereal Plane and trying to recover the totality of his memories, he thought of them as feeling “close,” and “just one more layer down.”
In Chapter 89, Dora learned from Lowls‘s journals about the existence of a section of the Dimension of Dreams known as the Dreamlands.
In Chapter 93, it was learned that the Bloodwind is able to travel there.
In Chapter 102, the Yellow King warned the party about spending too much time on the borderlands between the Dreamlands and the Dimension of Dreams.
In Chapter 110, the party reviewed the way dreaming 'works'–information they learned from The Chain of Nights.
First Reference: Chapter 10
Other Notable References: Chapter 22, Chapter 23, Act I Epilogue, Act II Interlude, Chapter 62, Chapter 73, Chapter 89, Chapter 92, Chapter 93, Chapter 102, Chapter 110, Chapter 142