(; older form: ) 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 Gaelic they are called daoine sìth (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. Etymology 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. The scholar David Fitzgerald conjectured that the word sídh was 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 Irish folklore In many Gaelic tales, the aos