Etiology (alternately aetiology, aitiology) is the study of causation. The term (deriving from the Greek words aitia = cause and logos = word/speech) is used in philosophy, physics and biology in reference to the causes of various phenomena. It is generally the study of why things occur, or even the reasons behind the way that things act.

In medicine in particular, the term means the occurrences, reasons, and variables of diseases or pathologies.

An aetiological myth is a myth intended to explain a name. E.g., the name Delphoi and its associated deity, Apollon Delphinios are explained in the Homeric Hymn which tells how Apollo carried Cretans over the sea in the shape of a dolphin to make them his priests. While there is an actual etymological connection between Delphoi and delphis (delphus means "womb"), many aetiological myths are based on popular etymology (see e.g. Amazons).

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