The Collaborative International Dictionary of English v.0.48:

Fibrinogen \Fi*brin"o*gen\, n. [Fibrin + -gen.] (Physiol. Chem.)
   An albuminous substance existing in the blood, and in other
   animal fluids, which either alone or with fibrinoplastin or
   paraglobulin forms fibrin, and thus causes coagulation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fibrinogen
    n 1: a protein present in blood plasma; converts to fibrin when
         blood clots [syn: fibrinogen, factor I]