- Noun. (biochemistry) 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.
- Noun. (context, nutrition) One of three major classes of food or source of food energy (4 kcal/gram) abundant in animal-derived foods (qualifier, ie: meat) and some vegetables, such as legumes. ''see carbohydrate and fat for the other two major classes''.