The Collaborative International Dictionary of English v.0.48:

vitamin K \vitamin K\ n. (1929) [from G. Koagulationsvitamin.]
   either of two fat-soluble vitamins (vitamin K1 or vitamin K2)
   that help clot blood by participating in the fromation of
   prothrombin.

   Syn: naphthoquinone.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

vitamin K
    n 1: a fat-soluble vitamin that helps in the clotting of blood
         [syn: vitamin K, naphthoquinone, antihemorrhagic
         factor]