Aortic aneurysm: An outpouching (a local widening) of the largest artery in the body, the aorta, involving that vessel in its course above the diaphragm (thoracic aortic aneurysm) or, more commonly, below the diaphragm (abdominal aortic aneurysm). The diagnosis of an aortic aneurysm can be straight forward or difficult. Around 1900 the eminent physician William Osler said: "There is no disease more conducive to clinical humility than aneurysm of the aorta." At the area of the aneurysm, there is typically a bulge and the wall is weakened and may rupture. Because of the volume of blood flowing under relatively high pressure through the aorta, a ruptured aneurysm of the aorta is a catastrophe.

The word "aneurysm" comes from the Greek "aneurysma" meaning "a widening."