1. A deep vein thrombosis (blood clot along the wall of a blood vessel), usually in the leg, caused by sitting immobile for long periods in a cramped aircraft seat.

Once movement begins again the clot can move to heart or lungs, causing rapid death.

2. A form of phlebitis (inflammation of a vein) in which a blood clot forms in the lower leg after prolonged immobility in a cramped space; such as, traveling in a confined space, like a coach seat on a crowded air flight.

It is known as economy class syndrom because so many travelers are turning up with phlebitis.