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
"a diet high in protein"
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.
"The laboratory analysis confirmed that the newly synthesized protein was indeed a massive chain of amino acids linked tightly by peptide bonds."
In plain English: A protein is a type of molecule made from chains of amino acids that builds and repairs your body's tissues.
"She added some chicken to her salad to get more protein."
Usage: Use protein to refer to essential nutrients found in foods like meat and beans that your body needs to build and repair tissues. Do not use it as a verb or to describe non-food items unless you are specifically discussing scientific molecular structures.
The word protein was suggested by chemist Jöns Jacob Berzelius in the early 19th century. It entered English through French and German versions of a term coined from Ancient Greek roots meaning "primary" or "first."