BLOOD STAGNATION. There are many risk factors for deep venous thrombosis (dvt) or blood clots including but not limited to: immobility, age over 50, some cancers, recent surgery, pregnancy, tobacco abuse, blood clotting disorders. The blood pools and forms a blood clot which then has a potential to move to the lungs and cause something potentially more damaging called a pulmonary embolism (pe).
Clot in veins. Deep vein clot (DVT) is usually treated with blood thinners to reduce risk of clot moving or increasing in size. Superficial vein clot is usually treated conservatively with anti-inflammatories.
Thromboembolism. That is a very general term like saying "car" could refer to lots of different models, venous thromboembolism refers to any venous clot from anywhere in the body that moved from where it formed (like in a leg vein) to somewhere else in the body (like the lungs).
Sludge. Muscle activity in the legs is what propels blood through the veins. Immobilization reduces vein blood flow. This may lead to stagnant blood flow or clotting. This is a classic risk factor for clot.