The term 'etiology' has two meanings - one is philosophical, and the other is medical.
Etiology: The Philosophical Term
Etiology is the study of causality - in other words, the reasons things happen.
The term is also used in mythology - for example, an etiological myth is one that tries to explain the origins of something.
Etiology: The Medical Definition
As both words come from the same Greek root-word, it's not surprising that both definitions are similar. In medicine, etiology is a term used to describe the study of the origins of pathological diseases or conditions.