Entry 1 sense Webster, 1913 vitamin K1 · IPA 01 n. one of the two K vitamins, (C31H46O2). It is fat-soluble and occurs naturally as the trans isomer. 1. one of the two K vitamins, (C31H46O2). It is fat-soluble and occurs naturally as the trans isomer. Syn. phylloquinone, phytonadione, antihemorrhagic vitamin.