Term relating to a disease or disorder that has no apparent cause. The origin of an idiopathic disease is unknown. Such diseases can arise spontanously, are not infectious and are typically limited to individual animals (or humans) in a population. 

An example of an idiopathic disorder is vitiligo; spontaneous depigmentation of small, circumscribed areas of skin and hair anywhere on the horse's body, but most frequently around the eyes and mouth.