Source : Webster's Revised Unabridged Dictionary (1913)

Thrombin \Throm"bin\, n. [See {Thrombus}.] (Physiol. Chem.)
   The fibrin ferment which produces the formation of fibrin
   from fibrinogen.

Source : WordNet®

thrombin
     n : an enzyme that acts on fibrinogen in blood causing it to
         clot