Vitamin

• substance that is essential for the proper metabolism and regulation of body processes; vitamins were so named because they were though [thought] originally to contain an amine radicle, which is incorrect
  Source: Noland, George B. 1983. General Biology, 11th Edition. St. Louis, MO. C. V. Mosby
• Organic substances that cannot be produced in the body but are essential for cellular functions and must be obtained from the diet.
  Source: Biotechnology Glossary
• A key nutrient that the body needs in small amounts to grow and stay strong. Examples are vitamins A, C, and E.