1. The study of causation.
2. Any study of causes, causation, or causality; as in philosophy, biology, or physics.
3. The study of the causes of diseases or medical disorders.
4. The cause or origin of a disease.

The word etiology is primarily used in medicine, where it refers to the science that deals with the causes or origins of diseases including the factors which produce or predispose toward a certain disease or disorder.

Today in medicine one often hears, or reads: "The etiology is unknown"; meaning: "We don't know the cause."

Aetiology is the preferred spelling in some countries, including the UK, whereas etiology, without an "a", is dominant in the United States.