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 (starting point) for clot formation (thrombosis) and embolization. As an aneurysm increases in size, the risk of rupture increases, which could lead to uncontrolled bleeding. 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.

The word is from Greek: ἀνεύρυσμα, aneurysma, "dilation", from ἀνευρύνειν, aneurynein, "to dilate".

Aneurysms are classified by type, morphology, or location.

A true aneurysm is one that involves all three layers of the wall of an artery (intima, media and adventitia). True aneurysms include atherosclerotic, syphilitic, and congenital aneurysms, as well as ventricular aneurysms that follow transmural myocardial infarctions (aneurysms that involve all layers of the attenuated wall of the heart are also considered true aneurysms).

A false aneurysm, or pseudoaneurysm, is a collection of blood leaking completely out of an artery or vein but confined next to the vessel by the surrounding tissue. This blood-filled cavity will eventually either thrombose (clot) enough to seal the leak or rupture out of the surrounding tissue.

Pseudoaneurysms can be caused by trauma that punctures the artery, such as knife and bullet wounds, as a result of percutaneous surgical procedures such as coronary angiography or arterial grafting, or use of an artery for injection.

Aneurysms can also be classified by their macroscopic shapes and sizes and are described as either saccular or fusiform. The shape of an aneurysm is not specific for a specific disease. The size of the base or neck is useful in determining the chance of for example endovascular coiling.

Saccular aneurysms, or "berry" aneurysms, are spherical in shape and involve only a portion of the vessel wall; they usually range from 5 to 20 cm (2.0 to 7.9 in) in diameter, and are often filled, either partially or fully, by a thrombus.Saccular aneurysms have a "neck" that connects the aneurysm to its main ("parent") artery, a larger, rounded area, called the dome.^{[citation needed]}