vitamin

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

• noun Any of various fat-soluble or water-soluble organic substances that are essential in minute amounts for normal growth and activity of living organisms. They are synthesized by bacteria and plants and are obtained by animals chiefly in their diet.
• noun A preparation containing one or more vitamins, often taken as a dietary supplement.

from the GNU version of the Collaborative International Dictionary of English.

• noun any of several organic chemical substances not synthesized by an animal and required in small quantities for normal metabolism, present in and obtained from the natural foods eaten by the animal. Human vitamins are also produced synthetically, and taken in pure form or in mixtures, as dietary supplements. Deficiencies of specific vitamins lead to certain specific disorders, such as scurvy, caused by an insufficiency of vitamin C (ascorbic acid).

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

• noun Any of a specific group of organic compounds essential in small quantities for healthy human growth, metabolism, development, and body function; found in minute amounts in plant and animal foods or sometimes produced synthetically; deficiencies of specific vitamins produce specific disorders.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of a group of organic substances essential in small quantities to normal metabolism