An aneurysm is a localized bulging or ballooning of the wall of an artery. It occurs when the arterial wall weakens, causing it to widen abnormally. The two main types are saccular aneurysms, which are spherical bulges, and fusiform aneurysms, which elongate a portion of the artery. Aneurysms can enlarge over time and rupture, causing life-threatening hemorrhage. Risk factors include atherosclerosis, infection, smoking, hypertension, and genetic conditions. Diagnosis involves physical exam and imaging tests. Treatment depends on the location and size of the aneurysm but may involve open or endovascular surgical repair to prevent rupture.