Thrombosis is a serious medical problem: a clot — a small lump of thickened blood — forms inside a blood vessel and blocks the blood from flowing normally. Your blood is supposed to clot when you cut yourself, to stop the bleeding. Thrombosis is that same useful process happening in the wrong place, inside a vessel where nothing should be blocking the way. It is dangerous because the part of the body past the block stops getting blood, and because the clot can break loose and travel to the lungs or the brain. You will hear it most often in two phrases: 'deep vein thrombosis' (often shortened to DVT), a clot in a deep vein of the leg, which is the one people worry about on long flights; and 'coronary thrombosis', a clot in the vessels of the heart. It is a doctor's word, not an everyday one — in normal conversation people say 'a blood clot'. The plural is thromboses.