A disorder resulting from inadequate blood flow in the vessels that supply the brain. Representative examples include cerebrovascular ischemia, cerebral embolism, and cerebral infarction. A spectrum of pathological conditions of impaired blood flow in the brain. They can involve vessels (ARTERIES; or VEINS) in the CEREBRUM, the CEREBELLUM, and the BRAIN STEM. Major categories include INTRACRANIAL ARTERIOVENOUS MALFORMATIONS; BRAIN ISCHEMIA; CEREBRAL HEMORRHAGE; and others.