An aneurysm is an enlarged and weakened section of an artery. An aneurysm is a serious health concern, because as it increases in size, it can rupture. Besides rupturing, aneurysms carry another risk. Blood clots can form in an aneurysm and block blood flow to parts of your body. Most aneurysms occur in your aorta, which is the largest artery in your body. It runs from your heart through your chest and abdomen.An aneurysm that occurs in your abdomen is called an abdominal aortic aneurysm (AAA). AAA is the most common type of aneurysm.