Vitamin K Definition

A fat-soluble vitamin, occurring in leafy green vegetables, tomatoes, and egg yolks, that promotes blood clotting and prevents hemorrhaging. It exists in several related forms, such as K1 and K2 .
American Heritage

A fat-soluble vitamin, synthesized constantly by intestinal bacteria in mammals and occurring in certain green vegetables, fish meal, etc., that promotes blood clotting and is required for the synthesis of prothrombin by the liver: the two naturally occurring varieties are vitamin K1, C31H46O2, found chiefly in alfalfa leaves, and vitamin K2, C41H56O2, found chiefly in fish meal: vitamin K3 (menadione) and vitamin K4 are prepared synthetically.

Webster's New World
A crystalline compound, C41 H56 O2 , isolated from putrefied fish meal or from various intestinal bacteria, used to stop hemorrhaging and in veterinary medicine as an antidote to certain poisons.
American Heritage
Any of several fat-soluble compounds found in liver and other animal products and in some fermented foods and synthesized in the body by intestinal bacteria.
American Heritage Medicine

(slang) The drug ketamine.