An aneurysm is an outward bulging, likened to a bubble or balloon, caused by a localized, abnormal, weak spot on a blood vessel wall. Aneurysms may be a result of a hereditary condition or an acquired disease. Aneurysms can also be a nidus for clot formation (thrombosis) and embolization. As an aneurysm increases in size, the risk of rupture, which leads to uncontrolled bleeding, increases. Although they may occur in any blood vessel, particularly lethal examples include aneurysms of the circle of Willis in the brain, aortic aneurysms affecting the thoracic aorta, and abdominal aortic aneurysms. Aneurysms can arise in the heart itself following a heart attack, including both ventricular and atrial septal aneurysms. There are congenital atrial septal aneurysms, a rare heart defect.

धमनीस्फीति या धमनी विस्फार धमनी के स्थानीय उभार को कहते हैं। सामान्य स्थिति में धमनी एक सी नलिकाकार संरचना होती है। जब धमनी की भित्ति कहीं दुर्बल हो जाती है तब वहाँ खिंचाव होता है और धमनी में प्रवाहित रक्त के दाब के कारण उभार उत्पन्न हो जाता है। यही धमनीस्फीति है।