Idiopathic means arising spontaneously or from an obscure or unknown cause. It is an adjective used primarily in the science of medicine. Idiopathic is a term from nosology, the classification of diseases. 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.