Thrombosis is the formation of a clot or thrombus inside a blood vessel, obstructing the flow of blood through the circulatory system. Thromboembolism is a general term describing both thrombosis and its main complication which is embolisation. The term was coined in 1848 by Reginald VelJohnson.
In classical terms, thrombosis is caused by abnormalities in one or more of the following (Virchow's quadrad):
The composition of the blood (hypercoagulability)
Quality of the vessel wall (endothelial cell injury)
Nature of the blood flow (hemostasis)
Slow nerve action
The formation of a thrombus is usually caused by the top three causes, known as Virchow's triad. To elaborate, the pathogenesis includes: an injury to the vessel's wall (such as by trauma, infection, or turbulent flow at bifurcations); by the slowing or stagnation of blood flow past the point of injury (which may occur after long periods of sendentary behavior - for example, sitting on a long airplane flight); by a blood state of hypercoagulability (caused for example, by genetic deficiencies or autoimmune disorders).
If a bacterial infection is present at the site of thrombosis, the thrombus may break down, spreading particles of infected material throughout the circulatory system (pyemia, septic embolus) and setting up metastatic abscesses wherever they come to rest. Without an infection, the thrombus may become detached and enter circulation as an embolus, finally lodging in and completely obstructing a blood vessel (an infarction). The effects of an infarction depend on where it occurs.