noun

• (biochemistry, countable) 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, uncountable) 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, countable) A food rich in protein, often a meat or meat substitute.