Real Dictionary

protein in American English
any of a large class of complex polymers consisting of long chains of polypeptides often bonded with nucleic acids, lipids, etc.: proteins are found in all cells, are essential to the diet of animals, are the basic components of cartilage, hair, skin, etc
adjective: of the nature of or containing protein
noun: any of numerous, highly varied organic molecules constituting a large portion of the mass of every life form and necessary in the diet of all animals and other nonphotosynthesizing organisms, composed of 20 or more amino acids linked in a genetically cont

protein in British English
noun: any of a large group of nitrogenous compounds of high molecular weight that are essential constituents of all living organisms. They consist of one or more chains of amino acids linked by peptide bonds and are folded into a specific three-dimensional shap