The Collaborative International Dictionary of English v.0.48:

Mucus \Mu"cus\ (m[=u]"k[u^]s), n. [L. mucus, muccus; cf. mucere
   to be moldy or musty, Gr. my`xa 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.
      [1913 Webster]

   2. (Physiol.) Any other animal fluid of a viscid quality, as
      the synovial fluid, which lubricates the cavities of the
      joints; -- improperly so used.
      [1913 Webster]

   3. (Bot.) A gelatinous or slimy substance found in certain
      algae and other plants.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mucus
    n 1: protective secretion of the mucus 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: mucus, mucous secretion]

Moby Thesaurus II by Grady Ward, 1.0:

120 Moby Thesaurus words for "mucus":
   albumen, antifriction, autacoid, batter, bile, black lead,
   bonnyclabber, butter, carrion, chalone, chyle, clabber, colostrum,
   cornstarch, corruption, cream, curd, dandruff, decay,
   digestive secretion, discharge, dough, egg white, endocrine,
   excrement, filth, foul matter, furfur, gall, gangrene,
   gastric juice, gaum, gel, gelatin, glair, gleet, glop, glue,
   gluten, glycerin, goo, gook, goop, graphite, gruel, gumbo, gunk,
   hormone, humor, ichor, intestinal juice, jam, jell, jelly,
   lachryma, lactation, leukorrhea, loblolly, lubricant,
   lubricating oil, lubricator, lymph, matter, mess, milk, molasses,
   mucilage, muck, mucor, obscenity, ordure, pancreatic juice, pap,
   paste, peccant humor, phlegm, plumbago, porridge, prostatic fluid,
   pudding, pulp, puree, purulence, pus, putrid matter, putty, rheum,
   rob, rot, saliva, salivary secretion, sanies, scurf, scuz, semen,
   semifluid, semiliquid, serous fluid, serum, silicone, size, slime,
   smut, snot, sordes, soup, sperm, starch, sticky mess, suppuration,
   sweat, syrup, tear, teardrop, tears, the whites, thyroxin, treacle,
   urine, wax