An aneurysm is a bulging section in the wall of a blood vessel that has become stretched out and thin. As the wall of the blood vessel bulges out, it becomes weaker and may burst or rupture, causing bleeding.

If an aneurysm in the brain bursts, it may cause a stroke. An aneurysm in a vessel that carries a lot of blood, such as the aorta, is often fatal if it bursts.