Aos sí (pronounced [iːsˠ ˈʃiː]; older form: aes sídhe [eːsˠ ˈʃiːə]) is the Irish name for a supernatural race in Celtic mythology – spelled sìth by the Scots, but pronounced the same – comparable to fairies or elves. They are said to descend from either fallen angels or the Tuatha Dé Danann, meaning the "People of Danu", depending on the Abrahamic or pagan tradition.

The aos sí are said to live underground in fairy forts, across the Western sea, or in an invisible world that co-exists with the world of humans. This world is described in the Lebor Gabála Érenn as a parallel universe in which the aos sí walk among the living.

In modern Irish the people of the mounds are also called daoine sí; in Scottish they are called daoine sídhe (in both cases, it means "people of the fairy mound").They are variously said to be the ancestors, the spirits of nature, or goddesses and gods.

In the Irish language, aos sí means "people of the mounds", as the "sídhe" in Irish are hills or burial mounds (consistent with Geoffrey Keating's suggestion that the aos sí came from the Land of the Dead). In modern Irish, the word is sí; in Scottish Gaelic, sìth; in Old Irish, síde, and the singular is síd.

By the time of the Celtic Revival, when the "Fairy Faith became a topic for English and English-language authors, sidhe in its various forms, with various meanings, became a loanword into English and took on a variety of, often inaccurate, meanings. In a number of later, English-language texts, the word sídhe is incorrectly used both for the mounds and the people of the mounds.

For example W. B. Yeats, writing in 1908, referred to the aos sí simply as "the sídhe". However sidh in older texts refers specifically to "the palaces, courts, halls or residences" of the otherworldly beings that supposedly inhabit them. The fact that many of these sídhe have been found to be ancient burial mounds has contributed to the theory that the aos sí were the pre-Celtic occupants of Ireland.

In the scholarly literature of the Popular Tales of Ireland (1880) the word sídh is conjectured to be synonymous with "immortal," and is compared with words such as sídsat "they wait/remain," síthbeo "lasting," sídhbuan "perepetual," and sídhbe "long life." In most of the tales concerning the sí a great age or long life is implied.

In folk belief and practice, the aos sí are often appeased with offerings, and care is taken to avoid angering or insulting them. Often they are not named directly, but rather spoken of as "The Good Neighbours", "The Fair Folk", or simply "The Folk". The most common names for them, aos sí, aes sídhe, daoine sídhe (singular duine sídhe) and daoine sìth mean, literally, "people of the mounds" (referring to the sídhe). The aos sí are generally described as stunningly beautiful, though they can also be terrible and hideous.

Aos sí are seen as fierce guardians of their abodes —whether a fairy hill, a fairy ring, a special tree (often a hawthorn) or a particular loch or wood. It is believed that infringing on these spaces will cause the aos sí to retaliate in an effort to remove the people or objects that invaded their homes.

Many of these tales contribute to the changeling myth in west European folklore, with the aos sí kidnapping trespassers or replacing their children with changelings as a punishment for transgressing.

The aos sí are often connected to certain times of year and hours; as the Gaelic Otherworld is believed to come closer to the mortal world at the times of dusk and dawn, the aos sí correspondingly become easier to encounter. Some festivals such as Samhain, Bealtaine and Midsummer are also associated with the aos sí.

Alternate Name:

• Aingil Anúabhair: "Proud angels"
• Daoine Uaisle': "The noble folk"
• Daoine maithe: "Good people"
• Deamhna Aerig: "Air demons"
• Dream Anúabhair: "Excessively proud
  
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• Sídhfir: "Immortal men"
• Sídheógaídhe: "Little immortals"
• Slúagh Cille: "Host of the churchyard"
• Slúagh na Marbh: "Host of the dead"
• Slúagh Sídhe: "Immortal host"
• Slúagh-Sídhe-Thúatha-Dé-Danann: "Immortal host of the Túatha Dé Danann"
• na Uaisle: "The noble" or "The highborn"