Thrombosis is the process of a blood clot, also known as a thrombus, forming in a blood vessel. This clot can block or obstruct blood flow in the affected area, as well as cause serious complications if the clot moves to a crucial part of the circulatory system, such as heart.
It is normal for the body to produce clotting factors like platelets and fibrin when a blood vessel is injured, to prevent an excessive loss of blood from the body. If this effect is over productive it can obstruct the flow of blood and form an embolus that moves around the blood stream.