Aneurysm, cardiac: Outpouching of an abnormally thin portion of the heart wall. An aneurysm is a localized widening (dilatation) of an artery, vein, or the heart. At the area of an aneurysm, there is typically a bulge and the wall is weakened and may rupture. The word "aneurysm" comes from the Greek "aneurysma" meaning "a widening." Heart aneurysms tend to involve the left ventricle because the blood there is under greatest pressure.