Prothrombin: A coagulation (clotting) factor that is needed for the normal clotting of blood. A cascade of biochemical events leads to the formation of the final clot. In this cascade, prothrombin is a precursor to thrombin.
Prothrombin is also known as thrombinogen and, in medical jargon, as pro. See also: Prothrombin mutation G20210A; and Prothrombin time.