The aos sí (pronounced [eːsˠ ˈʃiː]; older form aes sídhe [eːsˠ ˈʃiːə]) is the Irish term for a supernatural race in Irish mythology and Scottish mythology (where it is usually spelled Sìth, but pronounced the same), comparable to the fairies or elves.
They are said to live underground in fairy mounds, across the western sea, or in an invisible world that coexists with the world of humans. This world is described in the Lebor Gabála Érenn as a parallel universe in which the aos sí walk amongst the living. In the Irish language, aos sí means "people of the mounds" (the mounds are known in Irish as "the sídhe"). In modern Irish the people of the mounds are also called daoine sídhe [ˈd̪ˠiːnʲə ˈʃiːə]; in Scottish mythology they are daoine sìth. They are variously said to be the ancestors, the spirits of nature, or goddesses and gods.Some secondary and tertiary sources in English, including well-known and influential authors such as W. B. Yeats, refer to aos sí simply as "the sídhe" (lit. "mounds").