Aneurysm, thoracic: An aneurysm situated within the thorax (chest). See: Aneurysm, aortic. 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."