An aneurysm or aneurism (from Lang-grc????????? - aneurusma "dilation", from ?????????? - aneurunein "to dilate"), is a localized, blood-filled dilation (balloon-like bulge) of a blood vessel caused by disease or weakening of the vessel wall.

Aneurysms most commonly occur in arteries (artery) at the base of the brain (the circle of Willis) and in the aorta (the main artery coming out of the heart, a so-called aortic aneurysm). As the size of an aneurysm increases, there is an increased risk of rupture, which can result in severe hemorrhage, other complications or even death.