Thrombosis, deep vein (DVT): Blood clotting in the veins of the inner thigh or leg. In air travel, deep vein thrombosis is the "economy-class syndrome." deep vein thrombosis can be associated with any state of relative immobility, including prolonged sitting, following surgery, or with illness. Some deep vein thrombosis are associated with familial syndromes. 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.