"Moyamoya" syndrome or disease is the term used to define the presence of an abnormal vascular network which develops because of gradual stenosis (narrowing) of the arteries at the base of the brain (upper end of the internal carotid artery or beginning of its branches anterior cerebral artery or middle cerebral artery). The abnormal vascular network appears gradually, to compensate for the reduction in blood flow downstream from the stenoses in the arteries at the base of the brain. It often has a "cloudy" or "hazy puff ofsmoke" appearance which, in Japanese, is translated by "moyamoya".
The abnormal vascular network can develop when the arteries at the base of the skull narrow as a result of an already known general or localized disease. These patients are described as having the "Moyamoya syndrome". The term Moyamoya disease" is used when no specific disorder is associated with the arterial abnormalities.