This is the presence of a blood clot in the veins, often caused by slowing down of blood flow. Thrombosis can strike deep veins with the risk of pulmonary embolism. If the thrombus forms in a superficial vein (often varicose) the skin becomes red and hot, the vein is painful and develops hardness.
Generally there are no risks of pulmonary embolism in this case. The causes of deep venous thrombosis may be varied: from blood diseases to trauma or prolonged bed rest.