Aneurysm, dissecting: 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 in which the wall of an artery rips (dissects) longitudinally. This occurs because bleeding into the weakened wall splits the wall. Dissecting aneurysms tend to affect the thoracic aorta. They are particularly a major danger in Marfan's disease.