A cerebral aneurysm (also known as an intracranial or intracerebral aneurysm) is a weak or thin spot on a blood vessel in the brain that balloons out and fills with blood. Cerebral aneurysms can occur anywhere in the brain; very often they are found on the large arteries at the base of the brain at the points where arteries branch off. Although cerebral (brain) aneurysms are much more common in adults than in children, they can occur in anyone and at any age.
Some cerebral aneurysms, particularly those that are very small, do not bleed or cause other problems. Small aneurysms are less than 11 millimeters in diameter (about the size of a standard pencil eraser), larger aneurysms are 11–25 millimeters (about the size of a dime), and giant aneurysms are greater than 25 millimeters in diameter (more than the size of a quarter).