A wish is a hope or desire for something. Fictionally, wishes can be used as plot devices. In folklore, opportunities for "making a wish" or for wishes to "come true" or "be granted" are themes that are at times used.
In fiction a wish is a supernatural demand placed on the recipient's unlimited request. When it is the centre of a tale, the wish is usually a template for a morality tale, "be careful what you wish for"; it can additionally be a small part of a tale, in which case it is often used as a plot device. One can wish on a large number of things for example: wishing wells, dandelions when one blows the seeds or light them on fire, stars and much more. When one wishes in a well he or she throws money in, in the hope their wish comes true, the money normally goes to a charity.
A template for fictional wishes can be The Book of One Thousand and One Nights, specifically the tale of Aladdin, although in the tale of Aladdin the actual wishes were only part of the tale. Also, Aladdin's demands, while outrageous, were mainly variations on wealth (which is still often taken as the most common request).
Classically the wish provider is often a spirit, genie or similar entity, bound or constrained within a commonplace object (Aladdin's oil lamp for example) or a container closed with Solomon's seal. Releasing the entity from its constraint, usually by a few simple action, allows the object's possessor to make a wish.
The subservience of the extraordinarily powerful entity to the wisher can be explained in several ways. The entity might be grateful to be free of its constraint and the wish is a thank-you gift. The entity might be bound to obedience by its prison or a few additional item that the wisher possesses. The entity may, by its nature, be unable to exercise its powers without an initiator.
Other wish providers are a wide variety of, more or less, inanimate objects. W.W. Jacob's Monkey's Paw is an example of this. Piers Anthony puts a spin on this idea in Castle Roogna: a magic ring claims to grant wishes and then claims credit when a wish comes true, apparently from the unaided efforts of the characters—but every wish made on the ring sooner or later comes true.
Some wishes appear to be granted by nothing in particular. Snow White's mother's wish for a beautiful child might have been a coincidence, but the father's wish in The Seven Ravens transforms his seven sons into ravens, just as the mother's wish in The Raven transforms her daughter. This is common in a tale involving a person, male or female, wishing for a child, even one that's a hedgehog, or a sprig of myrtle, or no bigger than a hazel nut.
The number of wishes granted varies. Aladdin had an unlimited number in the original story, but was restricted to three wishes in the 1992 Disney film. As in the Charles Perrault tale The Ridiculous Wishes, three is the most common, but others might be granted to fit the constraints of the tale. Several authors have spun variations of the wish for more wishes theme, though a few disallow this as cheating.
In a large number of storeys the wording of the wish is extremely important. For example, characters often say, "I wish I was wealthy." This wording can be taken literally, the wish granted so that at one time the wisher was (used to be) wealthy but isn't any more. Saying, "I wish to be wealthy", then because "to be" refers to either the present or the future, they would become wealthy.
A common problem is the granter of the wish being either extremely literal or through malice granting the request in a manner designed to cause maximum distress (such as a request for wealth being granted through inheritance/insurance on the death of a loved one). Certain authors have additionally tried an "always on" approach: the careless use of the word "wish" in everyday conversation having, often unpleasant, consequences.
Some cultures have customs in which people are encouraged to "make a wish", such as blowing out the candles on a birthday cake, seeing a shooting star at night, tossing a coin into a wishing well or fountain or breaking the wishbone of a cooked turkey. Many believe such wishes can only come true if they're kept a secret from additional people. Others, on the additional hand, believe that wishes come true only if they're told to someone else.