Etiology (/ˌiːtiˈɒlədʒi/; alternatively spelled aetiology or ætiology) is the study of causation or origination. The word is derived from the Greek word αἰτιολογία (aitiología), meaning "giving a reason for" (from αἰτία (aitía) 'cause' and -λογία (-logía) 'study of').^[1] 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.^[2] The word is commonly used in medicine (pertaining to causes of disease or illness) and in philosophy, but also in physics, biology, psychology, political science, geography, cosmology, 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.

In medicine, the etiology of an illness or condition refers to the frequent studies to determine one or more factors that come together to cause the illness. Relatedly, when disease is widespread, epidemiological studies investigate what associated factors, such as location, sex, exposure to chemicals, and many others, make a population more or less likely to have an illness, condition, or disease, thus helping determine its etiology. Sometimes determining etiology is an imprecise process. In the past, the etiology of a common sailor's disease, scurvy, was long unknown. When large, ocean-going ships were built, sailors began to put to sea for long periods of time, and often lacked fresh fruit and vegetables. Without knowing the precise cause, Captain James Cook suspected scurvy was caused by the lack of vegetables in the diet. Based on his suspicion, he forced his crew to eat sauerkraut, a cabbage preparation, every day, and based upon the positive outcomes, he inferred that it prevented scurvy, even though he did not know precisely why. It took about another two hundred years to discover the precise etiology; the lack of vitamin C in a sailor's diet.

The following are examples of intrinsic factors:

• Inherited conditions, or conditions that are passed down to you from your parents. An example of this is hemophilia, a disorder that leads to excessive bleeding.
• Metabolic and endocrine, or hormone, disorders. These are abnormalities in the chemical signaling and interaction in the body. For example, Diabetes mellitus is an endocrine disease that causes high blood sugar.
• Neoplastic disorders or cancer where the cells of the body grow out of control.
• Problems with immunity, such as allergies, which are an overreaction of the immune system.^[3]

An etiological myth, or origin myth, is a myth intended to explain the origins of cult practices, natural phenomena, proper names and the like. For example, the name Delphi and its associated deity, Apollon Delphinios, are explained in the Homeric Hymn which tells of how Apollo, in the shape of a dolphin (delphis), propelled Cretans over the seas to make them his priests. While Delphi is actually related to the word delphus ("womb"), many etiological myths are similarly based on folk etymology (the term "Amazon", for example). In the Aeneid (published c. 17 BC), Virgil claims the descent of Augustus Caesar's Julian clan from the hero Aeneas through his son Ascanius, also called Iulus. The story of Prometheus' sacrifice trick at Mecone in Hesiod's Theogony relates how Prometheus tricked Zeus into choosing the bones and fat of the first sacrificial animal rather than the meat to justify why, after a sacrifice, the Greeks offered the bones wrapped in fat to the gods while keeping the meat for themselves. In Ovid's Pyramus and Thisbe, the origin of the color of mulberries is explained, as the white berries become stained red from the blood gushing forth from their double suicide.

• Backstory
• Bradford Hill criteria
• Correlation does not imply causation
• Creation myth
• Just-so story
• Just So Stories
• Pathology
• Pourquoi story
• Problem of causation
• Involution (esoterism)

• The dictionary definition of etiology at Wiktionary