A thrombosis appears when a clot forms inside a blood vessel. A clot that forms inside a vein is called a venous thrombus (plural thrombi). Venous thrombi most commonly form in the veins of a leg or arm. When a clot breaks loose, it enters the bloodstream and travels until it becomes trapped in a smaller blood vessel, which often happens in the lungs. This is called a pulmonary embolism.