1) A diverse group of organic nutrient substances required in trace amounts for normal metabolic function; many are enzyme cofactors or cofactor precursors.

2) A general term for a number of unrelated organic substances that occur in many foods in small amounts and that are necessary in trace amounts for the normal metabolic functioning of the body. They may be water-soluble or fat-soluble. (Dorland, 28th ed)

3) A key nutrient that the body needs in small amounts to grow and stay strong. Examples are vitamins A, C, and E.