In medicine, an embolism occurs when an object (the embolus, plural emboli) migrates from one part of the body (through the circulation) and cause(s) a blockage (occlusion) of a blood vessel in another part of the body.

This can be contrasted with a "thrombus" which is the formation of a clot within a blood vessel, rather than being carried from elsewhere.

Blood clots form the most common embolic material by far: other possible embolic materials include fat globules, air bubbles (an air embolism), septic emboli (containing pus and bacteria), or amniotic fluid.

Emboli often have more serious consequences when they occur in the so-called "end-circulation": areas of the body that have no redundant blood supply, such as the brain, heart, and lungs.

Assuming a normal circulation, a thrombus or other embolus formed in a systemic vein will always impact in the lungs, after passing through the right side of the heart. This forms a pulmonary embolism that can be a complication of deep-vein thrombosis.

In the rare cases where there is a congenital hole in the heart (or some other abnormality of the circulation), it is possible that an embolus from a systemic vein can cross into the arterial system and land anywhere in the body.

Emboli starting in the heart (from a thrombus in the left atrium secondary to atrial fibrillation or septic emboli from endocarditis) can cause emboli in any part of the body.

An embolus landing in the brain from either the heart or a carotid artery will likely cause a stroke.