A blood clot in one of the large veins, usually in a person’s leg or arm, is called a deep vein thrombosis or DVT. When a blood clot like this forms, it can partly or completely block the flow of blood or blood circulation in the body. If this clot is not treated, it can move or break off and travel to the lungs. A blood clot in the lung is called a pulmonary embolism or PE, and can cause death and requires immediate medical attention.