Abdominal aortic aneurysm: A ballooning or widening of the main artery (the aorta) as it courses down through the abdomen. At the point of the aneurysm, the aneurysm usually measures 3 cm or more in diameter. The aneurysm weakens the wall of the aorta and can end in the aorta rupturing with catastrophic consequences. As the diameter of the aorta increases, the chances of an abdominal aortic aneurysm rupturing rise. A measurement of 5 cm is often used to recommend surgery. Persons with AAA tend to be 60 or over. Men are 5 times more likely than women to have an AAA.