Deep vein thrombosis: A blood clot (thrombus) in a deep vein in the thigh or leg. The clot can break off as an embolus and make its way to the lung, where it can cause respiratory distress and respiratory failure.
Deep vein thrombosis is sometimes called the "economy-class syndrome." Even in young, healthy travelers, long stretches of time spent immobilized in the cramped seat of an aircraft with very low humidity sets the stage for formation of a blood clot in the leg. Abbreviated as DVT.