Abdominal aneurysm: An aneurysm situated within the abdomen (belly). 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." An aneurysm may involve the aorta, the largest artery in the body, as it courses down through the abdomen. Because of the great volume of blood flowing under high pressure in the aorta, rupture of an abdominal aortic aneurysm is a surefire catastrophe.