Idiopathic

Idiopathic is an adjective used primarily in medicine meaning arising spontaneously or from an obscure or unknown cause. From Greek ἴδιος, idios (one's own) + πάθος, pathos (suffering), it means approximately "a disease of its own kind". It is technically a term from nosology, the classification of disease. For some 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 (for example, focal segmental glomerulosclerosis, the majority of which are idiopathic); with other conditions, however, idiopathic cases account for a small percentage (for example, 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.

Popular references
In his book The Human Body, Isaac Asimov noted a comment about the term idiopathic made in the 20th edition of Stedman's Medical Dictionary: "A high-flown term to conceal ignorance". Similarly, in the American television show House, the title character remarks that the word is "from the Latin, meaning: 'We're idiots 'cause we can't figure out what's causing it.' "