adjective.
Of, relating to, or designating a disease having no known cause.
The Century Dictionary and Cyclopedia
In pathology, of or pertaining to a primary morbid state; not secondary or arising from any other disease: as, an idiopathic affection: opposed to symptomatic.
the GNU version of the Collaborative International Dictionary of English
adjective.
Pertaining to idiopathy; characterizing a disease arising primarily, and not in consequence of some other disease or injury; -- opposed to symptomatic, sympathetic, and traumatic.