A ballooning out of the wall of an artery, which in time can leak blood or rupture, causing massive bleeding.
An aneurysm is an abnormal bulge or "ballooning" in the wall of an artery. Arteries are blood vessels that carry oxygen-rich blood from the heart to other parts of the body. An aneurysm may grow and become large enough to burst, causing bleeding inside the body that is dangerous and often fatal. Aneurysms can occur in arteries in the brain, heart, intestine, neck, spleen, back of the knees and thighs, and in other parts of the body. Stroke is caused by the bursting of an aneurysm in the brain.