http://archive.rubicon-foundation.org/dspace/bitstream/123456789/3933/1/ASCP4057_Handout.pdf
Deep-vein thrombosis (also known as deep-venous thrombosis or DVT and colloquially as economy class syndrome) is the formation of a blood clot ("thrombus") in a deep vein. It commonly affects the leg veins, such as the femoral vein or the popliteal vein or the deep veins of the pelvis. There is a significant risk of the thrombus embolizing and traveling to the lungs causing a pulmonary embolism. Many people with a past of blood clots have pulmonary embolisms and do not know it.