A sac formed by dilated wall of an artery or vein and is filled with blood. There are three basic form: (a) fusiform, which is a bulging of the blood vessel due to a weakened wall; (b) sacculated, which is the formation of a sac on the side of the blood vessel; and (c) dissecting, which is characterized by blood flowing between the layers of the blood vessel wall, separating the layers.
A bulge in the vascular tree (usually arterial); if the vessel ruptures, hemorrhage or internal bleeding occurs and, unless treated promptly, death can result.