The Collaborative International Dictionary of English v.0.48:

Monk \Monk\, n. [AS. munuc, munec, munc, L. monachus, Gr. ?, fr.
   mo`nos alone. Cf. Monachism.]
   1. A man who retires from the ordinary temporal concerns of
      the world, and devotes himself to religion; one of a
      religious community of men inhabiting a monastery, and
      bound by vows to a life of chastity, obedience, and
      poverty. "A monk out of his cloister." --Chaucer.
      [1913 Webster]

            Monks in some respects agree with regulars, as in
            the substantial vows of religion; but in other
            respects monks and regulars differ; for that
            regulars, vows excepted, are not tied up to so
            strict a rule of life as monks are.   --Ayliffe.
      [1913 Webster]

   2. (Print.) A blotch or spot of ink on a printed page, caused
      by the ink not being properly distributed. It is
      distinguished from a friar, or white spot caused by a
      deficiency of ink.
      [1913 Webster]

   3. A piece of tinder made of agaric, used in firing the
      powder hose or train of a mine.
      [1913 Webster]

   4. (Zool.)
      (a) A South American monkey (Pithecia monachus); also
          applied to other species, as Cebus xanthocephalus.
      (b) The European bullfinch.
          [1913 Webster]

   Monk bat (Zool.), a South American and West Indian bat
      (Molossus nasutus); -- so called because the males live
      in communities by themselves.

   Monk bird(Zool.), the friar bird.

   Monk seal (Zool.), a species of seal (Monachus
      albiventer) inhabiting the Black Sea, the Mediterranean
      Sea, and the adjacent parts of the Atlantic.

   Monk's rhubarb (Bot.), a kind of dock; -- also called
      patience (Rumex Patientia).
      [1913 Webster]

WordNet (r) 3.0 (2006):

monk
    n 1: a male religious living in a cloister and devoting himself
         to contemplation and prayer and work [syn: monk,
         monastic]
    2: United States jazz pianist who was one of the founders of the
       bebop style (1917-1982) [syn: Monk, Thelonious Monk,
       Thelonious Sphere Monk]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "monk":
   Cape polecat, abbacomes, abbot, ape, ascetic, bar, beadsman, bear,
   brother, caloyer, cavy, celibataire, celibate, cenobite, chimp,
   chimpanzee, coenobite, conventual, conventual prior, coon, ferret,
   foumart, friar, glutton, grand prior, groundhog, guinea pig,
   hedgehog, hermit, hieromonach, lay abbot, lay brother, mendicant,
   misogamist, misogynist, monastic, monkey, mousehound, nun, opossum,
   palmer, pilgrim, pillar saint, pillarist, polecat, porcupine,
   possum, prairie dog, priest, prior, quill pig, raccoon, religieux,
   religious, single, skunk, stylite, unmarried, weasel, whistle-pig,
   wolverine, woodchuck, zoril