A fairy (also known as fay, fae; from faery, faerie, "realm of the fays") is a type of mythical being or legendary creature in European folklore, a form of spirit, often described as metaphysical, supernatural or preternatural.

Fairies resemble various beings of other mythologies, though even folklore that uses the term fairy offers many definitions. Sometimes the term describes any magical creature, including goblins or gnomes: at other times, the term only describes a specific type of more ethereal creature or sprite.

Much of the folklore about fairies revolves around protection from their malice. Although in modern culture they are often depicted as young, sometimes winged, humanoids of small stature, they originally were depicted quite differently: tall, radiant, angelic beings or short, wizened trolls being two of the commonly mentioned forms. One common theme found among the Celtic nations describes a race of diminutive people who had been driven into hiding by invading humans. When considered as beings that a person might actually encounter, fairies were noted for their mischief and malice.

Fairies are generally described as human in appearance and having magical powers. Their origins are less clear in the folklore, being variously dead, or some form of demon, or a species completely independent of humans or angels.

The concept of "fairy" in the narrow sense is unique to English folklore, conflating Germanic elves with influences from Celtic and Romance folklores, and later made "diminutive" according to the tastes of Victorian era "fairy tales" for children. The English term "fairy" can be applied to comparable beings in any of these cultures, more generally to similar beliefs in other European folklores or in comparative studies even worldwide. Fairie was in origin used adjectivally, meaning "enchanted" (as in fairie knight, fairie queene), but was used as a name for "enchanted" creatures from as early as the Late Middle English period. Fairies as the term is now understood were shaped in the literature of Romanticism during the Victorian era. Writers such as Sir Walter Scott and James Hogg were inspired by folklore which featured fairies.

Folklorists have suggested that their origin may lie partially in a conquered race living in hiding, or in religious beliefs that lost currency with the advent of Christianity.

Fairies are generally described as human in appearance and having magical powers. Their origins are less clear in the folklore, being variously dead, or some form of demon, or a species completely independent of humans or angels. Folklorists have suggested that their actual origin lies in a conquered race living in hiding, or in religious beliefs that lost currency with the advent of Christianity. These explanations are not necessarily incompatible, and they may be traceable to multiple sources.

Although in modern culture they are often depicted as young, sometimes winged, humanoids of small stature, they originally were depicted quite differently: tall, radiant, angelic beings or short, wizened trolls being two of the commonly mentioned forms. Diminutive fairies of one kind or another have been recorded for centuries, but occur alongside the human-sized beings; these have been depicted as ranging in size from very tiny up to the size of a human child. Even with these small fairies, however, their small size may be magically assumed rather than constant. Some fairies though normally quite small were able to dilate their figures to imitate humans.

Wings, while common in Victorian and later artwork of fairies, are very rare in the folklore; even very small fairies flew with magic, sometimes flying on ragwort stems or the backs of birds. Nowadays, fairies are often depicted with ordinary insect wings or butterfly wings.

Various animals have also been described as fairies. Sometimes this is the result of shape shifting on part of the fairy, as in the case of the selkie (seal people); others, like the kelpie and various black dogs, appear to stay more constant in form.

In some folklore fairies have green eyes and often bite. Though they can confuse one with their words, fairies cannot lie. They hate being told 'thank you', as they see it as a sign of one forgetting the good deed done, and, instead, want something that will guarantee remembrance.