Traveller's thrombosis is a thrombosis incurred by sitting. Everybody who flies on business or privately knows: it's cramped on a plane. Economy class passengers are allowed about 17 inches seat width and 30 inches legroom.
When aircraft passengers sit motionless for hours in aeroplanes, the blood circulation slows down. The dry air and the low air pressure on board are contributory factors. This is compounded by the bent sitting position in the groin and, above all, the bent or crossed legs. The kinked veins now have to work hard to pump the blood out of the feet back to the heart. The blood can pool in the legs and the watery components leak through the venous wall into the tissues.
The result: the feet and legs swell up. The blood can thicken and, if a vein is then blocked, a thrombosis may develop.