Thrombosis is the formation of a blood clot, i.e. the process of coagulation and clotting of the blood in a blood vessel. When thrombosis is complete, the result is a thrombus (or blood clot), which impedes the flow of the blood through the artery or vein where the clot is formed.

Thrombosis occurs during in either 1) injury to the blood vessel, where the process attempts to “fix” and “patch” the leaky vessel in order to prevent blood loss, or 2) under pathophysiological conditions. In short, thrombosis is a physiological process that occurs in response to blood vessel injury. However, it can occur in the absence of vessel injury, e.g. in individuals who are genetically susceptible to the formation of blood clots, or in individuals with high levels of inflammation.