Etiology (pronounced ; alternatively: aetiology or ætiology) is the study of causation or origination. The word is derived from the Greek αἰτιολογία (aitiología) "giving a reason for" (αἰτία, aitía, "cause"); and -λογία (-logía). More completely, etiology is the study of the causes, origins, or reasons behind the way that things are, or the way they function, or it can refer to the causes themselves. The word is commonly used in medicine (pertaining to causes of disease) and in philosophy, but also in physics, biology, psychology, government, geography, spatial analysis and theology in reference to the causes or origins of various phenomena.

In the past, when many physical phenomena were not well understood or when histories were not recorded, myths often arose to provide etiologies. Thus, an etiological myth, or origin myth, is a myth that has arisen, been told over time or written to explain the origins of various social or natural phenomena. For example, Virgil's Aeneid is a national myth written to explain and glorify the origins of the Roman Empire. In theology, many religions have creation myths explaining the origins of the world or its relationship to believers.