A thrombus is a blood clot that forms in a vein.

An embolus is anything that moves through the blood vessels until it reaches a vessel that is too small to let it pass. When this happens, the blood flow is stopped by the embolus. An embolus is often a small piece of a blood clot that breaks off (thromboembolus). It may also be fat, air, amniotic fluid, a tumor, or a foreign substance such as talc, iodine, cotton, or a tiny piece of catheter tube.