Deep vein thrombosis (DVT) is the formation of a blood clot (thrombus) in a deep vein, most commonly in the lower leg, thigh, or pelvis. It may obstruct blood flow and can be dangerous if part of the clot dislodges and travels to the lungs, causing a pulmonary embolism (PE). DVT and PE together are called venous thromboembolism (VTE). CDC definition