protein

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of a group of complex organic macromolecules that contain carbon, hydrogen, oxygen, nitrogen, and usually sulfur and are composed of one or more chains of amino acids. Proteins are fundamental components of all living cells and include many substances, such as enzymes, hormones, and antibodies, that are necessary for the proper functioning of an organism. They are essential in the diet of animals for the growth and repair of tissue and can be obtained from foods such as meat, fish, eggs, milk, and legumes.

from The Century Dictionary.

• noun A hypothetical substance formerly believed to be the essential nitrogenous constituent of food, and to exist in animal and vegetable albumin, fibrin, casein, and other bodies.
• noun Collectively, the nitrogenous components of food, sometimes excluding and sometimes including certain gelatinoid and amidic substances which are without nutritive value or possess it in an inferior degree.
• noun A trade-name for a dried preparation which consists essentially of the casein of milk: sold as a food material.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Physiol. Chem.) In chemical analysis, the total nitrogenous material in vegetable or animal substances, obtained by multiplying the total nitrogen found by a factor, usually 6.25, assuming most proteids to contain approximately 16 per cent of nitrogen.
• noun (Physiol. Chem.) any polymer of an amino acid joined by peptide (amide) bonds. Most natural proteins have alpha-amino acids as the monomeric constituents. All classical enzymes are composed of protein, and control most of the biochemical transformations carrie dout in living cells. They may be soluble, as casein, albumins, and other globular proteins, or insoluble (e. g. "structural proteins"), as collagen or keratin. "albumin", an older term for protein, is now used primarily to refer to certain specific soluble globular proteins found in eggs or blood serum, e.g. bovine serum albumin, the main soluble protein in teh serum of cattle, used as an enzymatically inert protein in biochemical research.
• noun (Bot.) See Crystalloid, n., 2.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of a large group of nitrogenous organic compounds that are essential constituents of living cells; consist of polymers of amino acids; essential in the diet of animals for growth and for repair of tissues; can be obtained from meat and eggs and milk and legumes