DVT (deep vein thrombosis): Blood clotting in the veins of the inner thigh or leg. In air travel, DVT is the "economy-class syndrome." Even in young, health travelers the long stretches immobilized in cramped seats in cabins with very low humidity set the stage for the formation of a thrombus (blood clot) in the lower leg. Blood clots can break off (as emboli) and makes their way to the lung where they have the potential of causing respiratory distress and respiratory failure.