Source : Webster's Revised Unabridged Dictionary (1913)

Mucus \Mu"cus\, n. [L. mucus, muccus; cf. mucere 8be moldy or
   musty, Gr. ? mucus, and Skr. muc to release. Cf. {Match} for
   striking fire, {Moist}, {Mucilage}.]
   1. (Physiol.) A viscid fluid secreted by mucous membranes,
      which it serves to moisten and protect. It covers the
      lining membranes of all the cavities which open
      externally, such as those of the mouth, nose, lungs,
      intestinal canal, urinary passages, etc.

   2. (Physiol.) Any other animal fluid of a viscid quality, as
      the synovial fluid, which lubricates the cavities of the
      joints; -- improperly so used.

   3. (Bot.) A gelatinous or slimy substance found in certain
      alg[ae] and other plants.

Source : WordNet®

mucus
     n : protective secretion of the mucous membranes; in the gut it
         lubricates the passage of food and protects the
         epithelial cells; in the nose and throat and lungs it can
         make it difficult for bacteria to penetrate the body
         through the epithelium [syn: {mucous secretion}]