The Collaborative International Dictionary of English v.0.48:

vitamin K2 \vitamin K2\, vitamins K2 \vitamins K2\n.
   one of the two K vitamins, actually a mixture of homologous
   fat-soluble substituted naphthoquinones (called
   menaquinones), (C11H7O2[C5H8]nH), where n may be 1 to 13
   but is mostly 7 to 9. The term is also used for synthetic
   compounds resembling vitamin K2 and having the same
   physiological action. The individual components of vitamin K2
   are also referred to by the number of isoprenyl units in the
   side chain (the number n in the formula), as for menadione,
   having no units at that position of the naphthaquinone ring,
   and also called vitamin K2(0) (and also called vitamin K3).
   --[MI11]

   Syn: menaquinones, antihemorrhagic vitamin..
        [WordNet 1.5 +PJC]