Etiology is a doctor’s word with two close jobs. First, the cause of a disease — what started it. Second, the scientific study of those causes. You will see it in hospitals, research papers, and textbooks. American spelling is etiology; British spelling often keeps an extra a near the front: aetiology. Though actually, outside medicine, academics sometimes use it more loosely for “the cause of this problem” — a war, a habit, a change in language. That use is real, but it still sounds very formal. For most learners it is safest as a medical word. Ordinary people just say “the cause” or “why it happened.” It does not mean the treatment, and it does not mean the list of symptoms.