The Collaborative International Dictionary of English v.0.48:

Protein \Pro"te*in\, n. [Gr. prw^tos first: cf. prwtei^on the
   first place.] (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.

   Note: In the 1913 dictionary, protein was defined as: "A body
         now known as alkali albumin, but originally considered
         to be the basis of all albuminous substances, whence
         its name."
         [1913 Webster + PJC]

   Protein crystal. (Bot.) See Crystalloid, n., 2.
      [1913 Webster]