noun. 1  Any of various fat-soluble or water-soluble organic substances that are essential in minute amounts for normal growth and activity of living organisms. 2  A preparation containing one or more vitamins, often taken as a dietary supplement.oun. 1  Any of various fat-soluble or water-soluble organic substances that are essential in minute amounts for normal growth and activity of living organisms. 2  A preparation containing one or more vitamins, often taken as a dietary supplement.