A disease is referred to as ideopathic if it normally arises spontaneously or has no known cause in a particular patient. It is usually used to differentiate a disease that does have a known cause when the usual or known causes could not possibly have led to the development of the disease in a specific patient.
For example, there are few known causes of ankylosing spondylitis, so most cases are referred to as ideopathic. Conversely, pulmonary fibrosis has several well known cases, and only infrequently is a case described as ideopathic.