This is an object or being that, in order to see it (or hear it, touch it, etc.), you have to already believe it's there. However, this doesn't change the fact that they are there, even if you have no way to sense them.
To the skeptic, when the door slammed on its own, it was "just the wind". When The Hero suddenly got his Heroic Second Wind, it was because they believed in themselves, and surely not the invisible MacGuffin everyone else believes in. However, the point to this trope is that the skeptic is wrong, because the story tells us the thing they don't believe in is there. The degree that the skeptic can't perceive the object or being may vary, such as being able to feel, hear or otherwise sense something, but not completely know what. Or they may not perceive the thing itself in any way (passing right through it, for example), but be able to detect its effects on the surroundings, such as other objects moving by themselves.
In philosophy, the problem this poses is called "The Invisible Gardener". If one person believes that some unseen person maintains a garden, but after waiting and watching, the other person neither sees, hears, or otherwise detects the presence of said gardener in any way, then to the second person, the gardener may as well not exist. Except, again, this is a story—so, we know that it does thanks to cinematography or description. In cases where we don't know, see Maybe Magic, Maybe Mundane.
This trope is in a similar vein to Gods Need Prayer Badly and Clap Your Hands If You Believe. Like in those tropes, belief in the metaphysical is key. However, unlike those tropes, the thing that's being imagined was there all along, but in a form that is useless to human senses. It isn't until you believe it's there that it finally makes itself manifest to you.
Compare Enlightenment Superpower, Power Born of Madness, Achievements in Ignorance, Imaginary Friend, Not-So-Imaginary Friend, Invisible to Normals and Living Dream. Contrast Your Mind Makes It Real where an illusion has to be observed before becoming real later on.
NOTE: Again, what makes this different from Clap Your Hands If You Believe is that believing in something isn't what causes it to exist. It existed all along and (usually) would continue to exist even if the other characters stopped believing. It would simply remain Invisible until belief returned.
- Fairies and elves are this way in Berserk. During one scene where Guts has kidnapped Farnese, Puck tries to interact with her, only for Farnese to not see him even though he's right before her eyes. Puck explains that this is because of a rigid upbringing that doesn't account for the existence of his kind. Those that believe in fairies, however, can see him with no problem.
- Yokai in GeGeGe no Kitarō are inly seen by humans who really believe in them willingly.
- In Higurashi No Naku Koro Ni, there is a deity called Oyashiro-sama that people believe in. They think a lot of things about it that simply aren't true. For instance, they call Oyashiro a guy, only the REAL Oyashiro is....a FEMALE. Not only that, some people, like Takano, believe Oyashiro-sama approved of the horrible deeds that the villagers committed in the past. In reality, Takano is the true monster, while Oyashiro-sama is nothing more than an innocent girl goddess who wishes for peace for her friends. However, for the majority of the series, Hanyuu is there, but invisible to everyone except for Rika and people who are at a high enough level on the Hinamizawa Syndrome.
- The title character in Drop Dead Fred is an Imaginary Friend who was sealed in a Jack-In-The-Box for many years, until his creator accidentally releases him. Presumably if he hadn't been sealed up he'd have been out in the world even if he had no longer been believed in.
- Erik the Viking: Harald Missionary, the only Christian in Erik's crew, cannot see the Dragon of the North Sea, the Edge of the World, and the fortress of Asgard, because he considers these things as superstitions. He is even able to walk right through the walls of Asgard, which are perfectly solid for his pagan companions.
- The banquet in Hook. Everyone starts gorging themselves around Peter, and he can't understand it. When they tell him to simply imagine the food, it finally appears. No, actually, judging by everyone's surprise after it happens, his imagination overrides theirs.
- In The Santa Clause, in the Workshop, Judy the elf states this to Scott, adding, "kids don't need to see this place to know that it's there. They just... know."
- The Chronicles of Narnia: Prince Caspian: When the Pevensie children are returned to Narnia for the first time, at first only Lucy sees Aslan. Edmund doesn't see Aslan, but he believes her (since she was right about Narnia the first time) but the older Peter & Susan veto Lucy, thinking that either she's fibbing or imagining things. But after they've been lost for hours they finally give in and let Lucy lead the way - Lucy is following Aslan. Slowly, Edmund & Peter start to see him. Eventually even Susan sees him, even though she sees him before she admits to it.
- Double Subverted in The Emperor's New Clothes by Hans Christian Andersen. A pair of con men sell the titular Emperor a set of "clothes" that are supposedly invisible to anyone unfit for his position or hopelessly stupid. It's subverted because of course there are no clothes, but double subverted because everyone pretends to see them because of social pressure and the fear of looking like an idiot.
- Unicorns in The Last Unicorn are visible as themselves only to those who believe they exist — otherwise they look like horses. This leads a witch who imprisons a unicorn for a side-show to create an illusionary second horn that everyone sees.
- In the Nellie and the Dragon books by Elizabeth Lindsay, nobody can see Gertrude the Dragon unless they already believe she exists. At first, that's just Nellie, but later her best friend also comes to believe. Gertrude can still interact invisibly with people who don't believe in her, which gets Nellie into quite a bit of trouble.
- The children's book O'Diddy by Jocelyn Stevenson is built on this torpe, combined with a dose of Accidental Nightmare Fuel: when an imaginary friend's child partner grows too old to believe in them, the imaginary friend is still around, but invisible and unable to interact with them in any way. The only way to avoid an And I Must Scream situation is to seek out another child to befriend. Or to do what O'Diddy does at the end, and convince the grown up child to believe again.
- Parzival: When Parzival and his pagan half-brother Feirefiz enter the Grail Castle, it turns out Feirefiz cannot see the Holy Grail because he is not a Christian. As soon as Feirefiz has renounced Jupiter and let himself be baptized, he can see the Grail.
- Lampshaded in Voyage of the Basset (and the film it inspired, Voyage of the Unicorn); the titular ship's motto, "Credendo vides", is translated in - universe as "By believing, one sees." In fact, a more accurate translation of the latin might be "you believe as you see".
- In Doctor Who, Perception Filters are explained to work to this way. If you don't want to/don't have a reason to believe there's an anachronistic blue police box on the street corner, a second floor on your house that shouldn't be there, etc., then you simply won't see anything. But if you do want to believe there's something there, the perception filter falters and you become aware of the thing it was shielding.
- Japanese Mythology has this system in place for the "kami", the gods/spirits/ancestors that control natural phenomena. In Shinto mythology, the kami are Anthropomorphic Personifications of natural phenomena, objects, or concepts that reside in an "invisible world" outside of human perception, but that is still part of the real world. In order to see them, the first step is to already believe they're there, but that's not the only requirement. Only the Pure of Heart will be able to fully see and interact with them.
- In Ōkami: The protagonist, Amaterasu, can be seen by the other characters but her facial markings can only be seen by those who still believe in the gods. Those who can't assume she's just an ordinary wolf. There are two sisters, the younger of whom believes in the gods (and can see Ammy's true form) while her older sister cannot. If you perform enough Celestial Brush powers for her, though, the older sister does start to believe, and will comment that she can see "faint" markings on Ammy.
- In the Ace Attorney series, a master of the Kurain channeling technical can host a dead person's spirit accurately and on command. During a channeling, the medium's physical body changes and she effectively becomes the other person. Phoenix Wright's assistants Maya and Pearl have done this in court, many times, yet very few seem to see the physical change (some, like Miles Edgeworth, seem actively resistant to seeing it due to a strong bias against spiritual powers).
- In Umineko: When They Cry, witches like Beatrice, and magic in general, can supposedly only be seen by those who genuinely believe in them in the human world. That's because they don't actually exist in the human world.
- Played with in the webcomic Count Your Sheep, where Katie's "imaginary friend" Ship is real, but can only be seen by certain people. At first that's Katie and Katie's mom Laurie (whose imaginary friend Ship was when she was Katie's age), but later the strip introduces Laurie's mom — who can see Ship, but repeatedly denies that she believes he exists even when she's talking to him and there's nobody else around — and Laurie's Cool Big Sis — who can't see Ship, even though she believes he exists.
- Played with in the Dexter's Laboratory episode "The Koos is Loose". Dee Dee has an imaginary friend named Koosalagoopagoop. Dexter is able to interact with him as well, but is skeptical of his existence. When Dexter, after constant pestering by Koosy, demands that he disappears forever by thinking him away, Koosy complies. It is only then that Dexter realizes that he likes Dee Dee's imaginary friend almost as much as she does. Later on, it's revealed that Koosy still exists, with Dee Dee actually travelling to his world (where it turns out that she's his imaginary friend) and Koosy later showing up as a construct of Bubbles' imagination.
- Mr. Hankey the Christmas Poo from South Park appears when enough people in the general vicinity believe in his existence.
- In one episode of Teen Titans Raven guards three super powered children all the while telling the oldest one her imaginary friend isn't real but at the end of the episode he's what saved all their lives.