Dream worlds are illusory locations in the Final Fantasy series; they are distinct from the real worlds, and are generally the manifestations of someone's dreams or wishes.
Unei, Guardian of the Dreamworld, can access the world of dream and summon Dreamworld stones to the real world. While mentioned, the Dreamworld is never visited by the party.
The Dreamscape is a dream world formed from the mind of Cyan Garamonde as a reflection of his pain for the death of his family. It is visited after the Dream Stooges Laragorn, Curlax, and Moebius enter Cyan's mind if the party rests in Doma Castle with Cyan in the World of Ruin. It is also inhabited by Wrexsoul, a demon feeding on Cyan's confusion and doubt.
Not strictly speaking a dream world, Cloud's Subconscious is nonetheless a surreal location inside the mind of a comatose Cloud Strife. It is visited by Tifa Lockhart following Cloud and her falling into the Lifestream after Ultimate Weapon's attack on Mideel. Cloud was mentally unstable following the experiments of the Jenova Project, and so has to descend into his own mind to find himself, and Tifa manages to enter his mind thanks to the Lifestream's effects. Since it is Cloud's inner mind, various versions of himself exist as memories from the past.
Cloud also meets Aeris within a dream after the events in the Temple of the Ancients, in which she tells him she is going to take care of Sephiroth. It is implied this scene is of Aeris speaking directly into Cloud's mind.
The flower field is a dreamlike location in Advent Children where Cloud converses with Aerith in dream visions. When he speaks with Aerith, she is able to guide him and make him realize he had no part in her death. Cloud also encounters Zack in a similar dream world in Advent Children Complete and he meets Aerith and Zack together in the same world after he is shot—at which point they both send him back to Aerith's Church.
Rinoa asks Zell what's going on when she sees Squall, Selphie and Quistis lose consciousness, and he tells her they probably went to the "dream world". The player's party is sent to inhabit the party of Laguna, Kiros and Ward during numerous flashback scenarios that always begin with the player party falling asleep. Unlike the other dreamscapes in other Final Fantasy games, the dreams in Final Fantasy VIII are actually a form of time travel.
When hunting for Chocographs some treasures transport Choco to the Chocobo's Dream World, where he receives new abilities: to cross reefs, mountains, oceans, and finally, the ability to fly. The Fat Chocobo, flanked by a quartet of chocobos of all colors, seems to grant these abilities. Choco seeks a home and companionship and the abilities given to him will help him find his home, Chocobo's Paradise, which requires Choco's flight ability to reach.
The Dream World can only be accessed by riding Choco and finding one of four special Chocographs, which emit a special gas when discovered. Zidane will fall asleep and Choco will be transported to the Dream World. The following Chocographs grant access to the Dream World: Healing Shore, Dawn Lagoon, Green Plains, and Mist Ocean.
Additionally, Memoria, the penultimate dungeon, can be considered a dream world considering it was born of and reflects the planet Gaia's history, including drawing several memories from the party. The areas drawn from the party's memories cannot be seen by the party members who were not present at the time, such as Quina not being able to see the destruction of Alexandria, as s/he was not with the party at that time.
Although not technically a separate world, Dream Zanarkand is a dream copy of the original Zanarkand preserved by the memories of its surviving citizens, whom Yu Yevon turned into fayth and placed on Mt. Gagazet, protected by a dense mist. From their memories Yu Yevon created the Dream Zanarkand and set it far out at sea.
To protect himself and his city while he summoned it, Yu Yevon amassed a great amount of pyreflies to form an armor around him; called Sin. Dream Zanarkand is covered by ocean on all sides, and its inhabitants, unaware of being mere dreams of the fayth, believe it to be the entire world. The Dream Zanarkand is preserved in its idealistic state with no war, and thus no summoners, and it's a city of light that never sleeps.
Inside Sin also appears to be a dream world of sorts, featuring unnatural architecture and entire landscapes with dream-themed names.
Yuna, while wearing Lenne's dressphere experiences a realistic dream of herself and Tidus escaping Bevelle troops. She is, in fact, experiencing Lenne's final memories, as the pyreflies imprinted with her memories were condensed into a sphere. According to the Final Fantasy X-2: International + Last Mission Ultimania "Four Puzzles" section, the part where Yuna dreams holding hands with Tidus and escaping never happened with Lenne and Shuyin, but is due to her own dream melding with Lenne's memories while she sleeps wearing the Songstress dressphere.
The Shrouded Land of Dynamis is a parallel dimension separate from Vana'diel, and cannot be entered by normal means. It is a dreamworld created by the terrestrial Avatar Diabolos, ruler of dreams, to escape from Vana'diel's fate. The Shadow Lord (known as the Dynamis Lord here), the Dark Kindred, the Forlorn Vanguards, Hydra Corps, and several others were all transported to or entered the realm at some point.
During the party's trek through the Pharos they enter rooms that transport them into an illusionary landscape where they must defeat a boss to break the illusion and find the true path. These are implied to be traps left behind by the Occuria, who themselves seem to live in an alternate reality of sorts. When the Occuria summon Lady Ashe to give her the Treaty-Blade, she is transported to a celestial location nobody else of the party can see.
After the second auralith's destruction, Vaan, Penelo, Filo, Kytes and Llyud end up in the World of Illusion, the world of the Yarhi created by Feolthanos's anima. After defeating Feolthanos, the Yarhi's dreamworld disappears along with the Yarhi themselves.
You're trapped in a dream that never ends, somewhere in the Void Beyond. If we don't help, you'll never wake up.
When a person perishes in the Void Beyond, they enter a world constructed entirely of the person's wishes or memories. If the person accepts this world as reality, they will remain trapped in the dream indefinitely. Some call this place the "Mirror World".
When Serah is stabbed by Caius in the Void Beyond, she is sent to a dream version of New Bodhum where everything is the way she wants it to be. She is awakened by the NORA members who have no memories of Noel or the meteorite. Serah is confused and heads into the NORA House and finds Snow, to whom she is married, living in a world where Lightning never disappeared.
When meeting with Lightning on the dock, the player is presented with two Live Trigger options: to accept the dream world as Serah's new reality, or to refuse the dream. Accepting the dream leads to the "Fate and Freedom" paradox ending, where Serah resumes her life in the eternal dream alongside Lightning and Snow, but with faint memories of Noel and Mog.
If Serah refuses the dream, Lightning and everyone else disappears and Serah is guided by a female voice to the meteorite crash site, where she reunites with Vanille and meets Fang. The two reveal they come to help her get out from the eternal dream and tell her that Noel needs her help to escape from his own dream.
After escaping her dream world with Fang and Vanille's help, Serah is transported to Noel's dream world, who was sent there in the same manner as Serah. Noel's dream world is a darker version of the Dying World at 700 AF, where everything has shades of gray and the sky is red. In Noel's dream world, Serah learns about his past, how he refused to kill Caius and take the man's place as Yeul's Guardian, how Yeul died in his arms after having a vision of a happy future where they'd be together, and how Noel went through Etro's gate and ended up in Valhalla.
After helping Noel realize that place is just a dream and defeating Gogmagog, they search for a way to leave the dream world and proceed with their journey.
Ivalice is a dream kingdom created by Mewt Randell after he bought the Gran Grimoire from a bookstore. Mewt, his friends, and the entire town was brought to this dream kingdom after Mewt uttered some magic words inside the Grimoire. It is based on the real Ivalice from the past. Five Crystals hidden in special folds in space maintain this dream world's existence. Laws play an important role in this world, being supervised by the Judges of the palace.
Rolan's Soul is a dreamlike location in The 4 Heroes of Light that appears to exist inside the mind of Rolan, a former Hero of Light. After defeating the monsters that inhabit Rolan's mind that has been corrupted by darkness, darkness that was trapped within him is unleashed on the world and sends the party back in time. In the past the party is able to enter Rolan's Soul again and stop him from being possessed by Lucifer, and thus freeing him from darkness.
The world seen in Confessions of the Creator is a dream world created by Shinryu. Shinryu trapped Cid of the Lufaine here as punishment for interfering in the thirteenth and final cycle of the war. In this world, the cycles of war continued until Chaos became Feral Chaos and destroyed all summoned warriors and almost killed Cosmos. Cid intervened to seal Feral Chaos in the northern continents. Through an unknown method, five warriors (the player's party) are brought into the dream world and destroy Feral Chaos, freeing Cid.
Dream world is a commonly used plot device in fictional works, most notably in science fiction and fantasy fiction. The use of a dream world creates a situation whereby a character (or group of characters) is placed in a marvelous and unpredictable environment and must overcome several personal problems to leave it.