Congenital or acquired abnormal outpouching of an intracranial blood vessel wall. Saccular (berry) aneurysms are the most common variant, and tend to form at arterial branch points near the base of the brain. Rupture results in SUBARACHNOID HEMORRHAGE or INTRACRANIAL HEMORRHAGES. Giant aneurysms (>2.5 cm in diameter) may compress adjacent structures, including the OCULOMOTOR NERVE (see also OCULOMOTOR NERVE DISEASES). (From Adams et al., Principles of Neurology, 6th ed, p841).