DVT is short for deep vein thrombosis. That means a blood clot has formed in a deep vein — a vein that sits well under the skin, not one you can easily see. It is usually in the leg, though it can happen in the arm. Blood cannot pass easily, so the leg may swell, feel hot, or hurt. Doctors watch it closely because a piece of the clot can break off and travel to the lungs, which is dangerous. People say the three letters D-V-T, not as one word. You hear it in hospitals, on medicine sheets, and in talk about long flights or sitting still for hours. It is a doctor's word, though actually ordinary people now use it too after long plane rides.