Idiopathic is a medical adjective that indicates that a recognized cause has not yet been established. The combination of Greek roots means approximately "a disease of its own kind."
It is technically a term from nosology, the classification of disease. For most medical conditions, one or more causes are somewhat understood, but in a certain percentage of people with the condition, the cause may not be readily apparent or characterized. In these cases, the origin of the condition is said to be "idiopathic."
With some medical conditions, the medical community cannot establish a root cause for a large percentage of all cases (e.g. scoliosis, fully half of which is idiopathic); with other conditions, however, idiopathic cases account for a small percentage (e.g., pulmonary fibrosis). As medical and scientific advances are made with relation to a particular condition or disease, more root causes are discovered, and the percentage of cases designated as idiopathic shrinks.