The Collaborative International Dictionary of English v.0.48:

Peptone \Pep"tone\, n. [Gr. pepto`s cooked.] (Physiol. Chem.)
   (a) The soluble polypeptides produced by hydrolysis of
       protein; specifically the soluble peptides into which
       food is transformed by the action of the gastric and
       pancreatic juices. Peptones are also formed from protein
       matter by the action of boiling water and boiling dilute
       acids.
   (b) Collectively, in a broader sense, all the products
       resulting from the solution of proteinaceous matter in
       either gastric or pancreatic juice. In this case,
       however, intermediate products (albumose bodies), such as
       antialbumose, hemialbumose, etc., are mixed with the true
       peptones. Also termed albuminose.
       [1913 Webster + PJC]

   Note: Pure peptones are of three kinds, amphopeptone,
         antipeptone, and hemipeptone, and, unlike most
         proteins, are not precipitated by saturating their
         solutions with ammonium sulphate.
         [1913 Webster]

WordNet (r) 3.0 (2006):

peptone
    n 1: any of various water-soluble compounds that form by
         hydrolysis in the digestion of proteins to amino acids