Prothrombin is a protein that is important for blood clotting. Synonyms for prothrombin include clotting factor II, thrombinogen, and coagulation factor II. Other synonyms include factor IIa, thrombin precursor, and prothrombin complex. Prothrombin is converted to thrombin through a complex series of reactions involving other clotting factors. Thrombin then plays a key role in the final stages of the clotting process, converting fibrinogen to fibrin to form a stable clot. Understanding the synonyms for prothrombin is important when studying blood clotting disorders or when working in the medical field.