The Collaborative International Dictionary of English v.0.48:

Thrombin \Throm"bin\, n. [See Thrombus.] (Biochem.)
   The enzyme which converts fibrinogen into fibrin.
   [Webster 1913 Suppl. +PJC]
   [1913 Webster]

WordNet (r) 3.0 (2006):

thrombin
    n 1: an enzyme that acts on fibrinogen in blood causing it to
         clot