Aneurysm, venous: An aneurysm involving a vein. As opposed to an arterial aneurysm or a cardiac aneurysm. 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."