Thrombosis refers to abnormal, life-threatening blood clots that form in the artery or vein. A clot in the vein (usually in the leg or pelvis) is known as a deep vein thrombosis (DVT) and a clot that breaks off and travels to the lungs is known as a pulmonary embolism (PE). Together, DVT and PE make up venous thromboembolism (VTE).