An aneurysm is a permanent 'dilatation' or 'ballooning' of an artery to at least 150% of its normal size.
Aneurysms can affect the aorta, the major artery of the body running through the chest and abdomen. An abdominal aortic aneurysm (AAA) is the most common type. The thoracic aorta (in the chest) and thoracoabdominal aorta (involving the chest and abdomen) are less commonly involved.
Aneurysms can also involve the popliteal arteries (behind the knees), the femoral arteries (in the groins), the visceral arteries (supplying the abdominal organs), and the renal arteries (supplying the kidneys).