Etiology is the study of causation or origination, and it refers to the cause or causes of a disease or abnormal condition. In medicine, etiology is used to determine one or more factors that come together to cause an illness. Etiology can also refer to the causes themselves, and it is commonly used in philosophy, physics, biology, psychology, government, geography, spatial analysis, and theology in reference to the causes or origins of various phenomena.

The etiology of an illness or condition can be intrinsic, extrinsic, or idiopathic. Intrinsic etiology refers to causes that come from within the body, such as genetic mutations or hormonal imbalances. Extrinsic etiology refers to causes that come from outside the body, such as environmental factors or infections. Idiopathic etiology refers to causes that are unknown.

Understanding the etiology of a disease is important in developing effective treatments and preventive measures. 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 a disease.