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Definition of Factor iii
1. Noun. An enzyme liberated from blood platelets that converts prothrombin into thrombin as blood starts to clot.
Medical Definition of Factor iii
1. In the clotting of blood, tissue factor or thromboplastin; it initiates the extrinsic pathway by reacting with factor VII and calcium to form factor VIIa. See: thromboplastin. (05 Mar 2000)