The Collaborative International Dictionary of English v.0.48:

Mucedin \Mu*ce"din\, n. [From Mucus.] (Bot. Chem.)
   A yellowish white, amorphous, nitrogenous substance found in
   wheat, rye, etc., and resembling gluten; -- formerly called
   also mucin.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mucin \Mu"cin\ (m[=u]"s[i^]n), n. [From Mucus.]
   1. (Bot. Chem.) See Mucedin. [Obs.]
      [1913 Webster]

   2. (Physiol. Chem.) Any of a class of high molecular weight
      glycoproteins which are contained in mucus, and give to
      the latter secretion its peculiar ropy character. They are
      found in all the secretions from mucous glands, and also
      between the fibers of connective tissue, as in tendons.
      They form viscous solutions and serve to provide
      lubrication for movement within body cavities, and some
      protection of surfaces. See Illust. of Demilune.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

mucin
    n 1: a nitrogenous substance found in mucous secretions; a
         lubricant that protects body surfaces