An aneurysm is an abnormal dilatation (enlargement) in an artery. The dilatation occurs because of a weakness in the wall of the artery. These weak areas are subject to rupture, with disastrous consequences. Besides rupturing, these enlargements may cause pressure on other organs or on nerves, or they may develop a clot, called a thrombosis. The risk of the aneurysm varies with its location and the likelihood that it may rupture. Size is an indication of the likelihood of rupture. Larger aneurysms are at greater risk of rupture.