An aneurysm or aneurism (from - aneurusma "dilation", from ἀνευρύνειν - aneurunein "to dilate"), is a localized, blood-filled dilation (balloon-like bulge) of a blood vessel caused by disease or weakening of the vessel wall. Aneurysms most commonly occur in arteries at the base of the brain (the circle of Willis) and in the aorta (the main artery coming out of the heart, a so-called aortic aneurysm). As the size of an aneurysm increases, there'… (More on Aneurysms)