vitamin K

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.
• noun A crystalline compound, C₄₁H₅₆O₂, isolated from putrefied fish meal or from various intestinal bacteria, used to stop hemorrhaging and in veterinary medicine as an antidote to certain poisons.
• noun A yellow viscous oil, C₃₁H₄₆O₂, found in leafy green vegetables or made synthetically, used by the body in the synthesis of prothrombin and in veterinary medicine as an antidote to certain poisons.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Either of several related fat-soluble vitamins, found in leafy green vegetables, essential for blood clotting.
• noun slang The drug ketamine.