Brain aneurysms, also called cerebral or intracranial aneurysms, are balloon-like outpouchings of the arteries in the brain. They arise from a weak point in the wall of the artery and enlarge over time as a result of the pressure from within the artery. They have thin walls, which do not have the structure of normal arterial walls.

Their primary danger is that they may rupture, bleed into the brain, and have potentially disastrous consequences. This type of bleeding is known as subarachnoid hemorrhage.