n. one of the two K vitamins, (C31H46O2). It is fat-soluble and occurs naturally as the trans isomer. Chemically it is 2-methyl-3-phytyl-1, 4, -naphthoquinone. Syn. -- phylloquinone, phytonadione, antihemorrhagic vitamin.. [ WordNet 1.5 +PJC ]