Protein meaning : Any of numerous large, complex naturally-produced molecules composed of one or more long chains of amino acids, in which the amino acid groups are held together by peptide bonds. (nutrition) One of three major classes of food or source of food energy (4 kcal/gram) abundant in animal-derived foods (i.e. meat) and some vegetables, such as legumes. (nutrition) A food rich in protein, often a meat or meat substitute.