A thrombus is a blood clot that has formed inside an intact blood vessel – as distinct from one that has form to seal the wall of a blood vessel after injury (see blood clotting).

A thrombus is life-threatening if it grows to obstruct the blood supply to an organ such as the heart or brain. Even a thrombus in a less vital blood vessel can be dangerous because it can produce gangrene in part of an organ or extremity served by the blood vessel, or lead to embolism, in which a fragment of the thrombus breaks off and is carried to obstruct the blood circulation elsewhere.