Source : Webster's Revised Unabridged Dictionary (1913)

Protein \Pro"te*in\, n. (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.

Protein \Pro"te*in\, n. [Gr. prw^tos first: cf. prwtei^on the
   first place.] (Physiol. Chem.)
   A body now known as alkali albumin, but originally considered
   to be the basis of all albuminous substances, whence its
   name.

Source : WordNet®

protein
     n : 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"