An aneurysm or aneurism (from Greek: ἀνεύρυσμα, aneurysma, "dilation", from ἀνευρύνειν, aneurynein, "to dilate") is a localized, blood-filled balloon-like bulge in the wall of a blood vessel.Aneurysms can occur in any blood vessel, with examples including aneurysms of the circle of Willis in the brain, aortic aneurysms affecting the thoracic aorta, and abdominal aortic aneurysms. Aneurysms can also occur within the heart itself.As an aneurysm increases in size, the risk of rupture increases.