What is the term for the widening of a blood vessel that can lead to a blood sac?

The term that describes the widening of a blood vessel, which can result in the formation of a blood sac, is aneurysm. An aneurysm occurs when there is a localized dilation or bulging in the wall of a blood vessel, often due to an area of weakness in the vessel wall. This bulging can lead to potential complications, such as rupture or blood clots, if not monitored or treated.

Stenosis refers to the narrowing of a blood vessel, which is the opposite of what an aneurysm describes. Thrombosis involves the formation of a blood clot within a blood vessel, which can obstruct blood flow but does not describe a dilation or widening. Vasodilation refers to the process of blood vessels widening due to relaxation of the muscular walls, but it is not necessarily associated with the formation of a blood sac like an aneurysm. Thus, in the context of the question, aneurysm is the most appropriate term.