Deep vein thrombosis (DVT) refers to a blood clot that forms in a vein deep in the body, most frequently in the lower leg or thigh. While the clot sometimes will cause no symptoms in the person who has it, the clot or part of it may dislodge and travel to other areas of the body, especially the lung, where it may block an artery. This type of blockage is known as a pulmonary embolism. Stroke and heart attack are other potential consequences if the clot lodges in the brain or heart. According to the Surgeon General, between 350,000 and 600,000 Americans develop DVT or a pulmonary embolism each year. Together, they contribute to at least 100,000 deaths a year.
Basically, any activity that requires you to stay in a certain position for a long period of time adds to the risk of DVT.