What is an aortic aneurysm?

Weak wall. The aorta is a living pipe. It has 3 layers, the inner layer or intima, the middle layer or media were some muscle like cells dwell and the adventitia rich in collagen fibers. The process of arteriosclerosis may result in slow death of the media which then gets replaced by collagen scar. The pressure inside the aorta and the wall weakness will result in dilatation of the aorta and it could burst.
Large aorta. An aortic aneurysm is an aorta which is larger than it should be. This is typically due weakness of the walls of the aorta. This weakness can be secondary to degenerative change, congenital (born with it) abnormalities of the wall of the aorta, or trauma.