Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. (Physiol. Chem.) An albuminoid substance which is
      contained in mucus, and gives to the latter secretion its
      peculiar ropy character. It is found in all the secretions
      from mucous glands, and also between the fibers of
      connective tissue, as in tendons. See Illust. of
      {Demilune}.

Source : WordNet®

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