Aetiology: The study of the causes. For example, of a disorder.
Today in medicine one hears (or reads) that "the aetiology is unknown." Translation -- we don't know the cause.
Aetiology is the preferred spelling in some countries, including the UK, whereas "etiology" without an "a" has taken over in the US. The word "aetiology" comes from the Greek "aitia", cause + "logos", discourse.