Sylph (also called sylphid) is a mythological creature in the Western tradition. The term originates in Paracelsus, who describes sylphs as invisible beings of the air, his elementals of air. There is no substantial mythos associated with them.
Alchemy and literature
The first mainstream western discussion of sylphs comes with Alexander Pope. In Rape of the Lock, Pope satirizes French Rosicrucian and alchemical writings when he invents a theory to explain the sylph. In a parody of heroic poetry and the "dark" and "mysterious" literature of pseudo-science, and in particular the sometimes esoterically Classical heroic poetry of the 18th century in England and France, Pope pretends to have a new alchemy, in which the sylph is the mystically, chemically condensed humors of peevish women. In Pope's poem, women who are full of spleen and vanity turn into sylphs when they die because their spirits are too full of dark vapors to ascend to the skies. Belinda, the heroine of Pope's poem, is attended by a small army of sylphs, who foster her vanity and guard her beauty. This is a parody of Paracelsus, inasmuch as Pope imitates the pseudo-science of alchemy to explain the seriousness with which vain women approach the dressing room. In a slight parody of the divine battle in John Milton's Paradise Lost, when the Baron of the poem attempts to cut a lock of Belinda's hair, the sylphs interpose their airy bodies between the blades of the scissors (to no effect whatsoever). The chief sylph in "The Rape of the Lock" has the same name as Prospero's servant in Shakespeare's The Tempest: Ariel.
Willow, in Terry Brooks' Magic Kingdom of Landover series is a sylph and the wife of protagonist Ben Holiday. She is the daughter of the River Master and a wood elemental, giving her pale green skin and emerald hair. Her dual nature is reflected in the fact that she must transform into a Willow tree once every 21 days to maintain her life force. She has a tense and distant relationship with her father, as her existence serves as a permanent reminder to him of the brief relationship that he desires to reclaim, but never can. And so it is to her mother that she turns for guidance.
Because of their association with the ballet La Sylphide, where sylphs are identified with fairies and the medieval legends of fairyland, as well as a confusion with other "airy spirits" (e.g., in William Shakespeare's A Midsummer Night's Dream), a slender girl may be referred to as a "sylph".
"Sylph" has passed into general language as a term for minor spirits, elementals, or faeries of the air. Fantasy authors will sometimes employ sylphs in their fiction. Sylphs could create giant artistic clouds in the skies with their airy wings.