Aneurysm, AN yuh rihz uhm, is a balloonlike bulge that forms in a weakened area of the wall of an artery or vein. The most dangerous aneurysms are those that form in arteries, especially the arteries of the brain and the aorta, the main artery leading from the heart. Most aneurysms result from atherosclerosis, a disease caused by cholesterol build-up in artery walls. Other causes of aneurysms include genetic disorders or other defects present at birth.