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