a substance released from blood platelets and injured body tissues that assists in the clotting of the blood by initiating the conversion of prothrombin to thrombin
See thromboplastin in American Heritage Dictionary 4
(thrŏmˌbō-plăsˈtĭn)
noun
A protease that converts prothrombin to thrombin in the early stages of blood clotting. Also called thrombokinase.