n 1  :  either of two naturally occurring fat-soluble vitamins that are essential for the clotting of blood because of their role in the production of prothrombin in the liver and that are used in preventing and treating hypoprothrombinemia and hemorrhage:  a  :  a yellow oily naphthoquinone C₃₁H₄₆O₂ that is obtained esp. from alfalfa or made synthetically and that has a fast, potent, and prolonged biological effect, is effective orally, and is useful esp. in treating hypoprothrombinemia induced by anticoagulant drugs - called also phylloquinone , phytonadione , vitamin K₁  - see MEPHYTON   b  :  a pale yellow crystalline naphthoquinone C₄₁H₅₆O₂ that is obtained esp. from putrefied fish meal and is synthesized by various bacteria (as in the intestines of humans and higher animals) and that is much more unsaturated than vitamin K₁ and slightly less active biologically - called also menaquinone , vitamin K₂   2  :  any of several synthetic compounds that are closely related chemically to vitamins K₁ and K₂ but are simpler in structure and that have similar biological activity : esp  :  MENADIONE .
 
 
 
Similar sounding terms:  vitamin C  vitamin G  vitamin K₁  vitamin K₃ 
 
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