A deep vein thrombosis (DVT) is a blood clot that forms in a deep vein, usually in the leg. The deep veins in the leg are blood vessels which go through the calf and thigh muscles, and are not those which you can see just below the skin.
This is a companion discussion topic for the original article at https://patient.info/allergies-blood-immune/deep-vein-thrombosis-leaflet/preventing-dvt-when-you-travel