The Collaborative International Dictionary of English v.0.48:

Mask \Mask\ (m[.a]sk), n. [F. masque, LL. masca, mascha, mascus;
   cf. Sp. & Pg. m['a]scara, It. maschera; all fr. Ar. maskharat
   buffoon, fool, pleasantry, anything ridiculous or mirthful,
   fr. sakhira to ridicule, to laugh at. Cf. Masque,
   Masquerade.]
   1. A cover, or partial cover, for the face, used for disguise
      or protection; as, a dancer's mask; a fencer's mask; a
      ball player's mask.
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   2. That which disguises; a pretext or subterfuge.
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   3. A festive entertainment of dancing or other diversions,
      where all wear masks; a masquerade; hence, a revel; a
      frolic; a delusive show. --Bacon.
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            This thought might lead me through the world's vain
            mask.                                 --Milton.
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   4. A dramatic performance, formerly in vogue, in which the
      actors wore masks and represented mythical or allegorical
      characters.
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   5. (Arch.) A grotesque head or face, used to adorn keystones
      and other prominent parts, to spout water in fountains,
      and the like; -- called also mascaron.
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   6. (Fort.)
      (a) In a permanent fortification, a redoubt which protects
          the caponiere.
      (b) A screen for a battery.
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   7. (Zool.) The lower lip of the larva of a dragon fly,
      modified so as to form a prehensile organ.
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   8. A person wearing a mask; a masker.

            The mask that has the arm of the Indian queen. --G.
                                                  W. Cable.
      [Webster 1913 Suppl.]

   9. (Sporting) The head or face of a fox.

   Mask house, a house for masquerades. [Obs.]

   Death mask, a cast of the face of a dead person.
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The Collaborative International Dictionary of English v.0.48:

Mask \Mask\, v. t. [imp. & p. p. Masked; p. pr. & vb. n.
   Masking.]
   1. To cover, as the face, by way of concealment or defense
      against injury; to conceal with a mask or visor.
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            They must all be masked and vizarded. --Shak.
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   2. To disguise; to cover; to hide.
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            Masking the business from the common eye. --Shak.
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   3. (Mil.)
      (a) To conceal; also, to intervene in the line of.
      (b) To cover or keep in check; as, to mask a body of
          troops or a fortress by a superior force, while some
          hostile evolution is being carried out.
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The Collaborative International Dictionary of English v.0.48:

Mask \Mask\, v. i.
   1. To take part as a masker in a masquerade. --Cavendish.
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   2. To wear a mask; to be disguised in any way. --Shak.
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WordNet (r) 3.0 (2006):

mask
    n 1: a covering to disguise or conceal the face
    2: activity that tries to conceal something; "no mask could
       conceal his ignorance"; "they moved in under a mask of
       friendship"
    3: a party of guests wearing costumes and masks [syn:
       masquerade, masquerade party, masque, mask]
    4: a protective covering worn over the face
    v 1: hide under a false appearance; "He masked his
         disappointment" [syn: dissemble, cloak, mask]
    2: put a mask on or cover with a mask; "Mask the children for
       Halloween" [ant: unmask]
    3: make unrecognizable; "The herb masks the garlic taste"; "We
       disguised our faces before robbing the bank" [syn:
       disguise, mask]
    4: cover with a sauce; "mask the meat"
    5: shield from light [syn: mask, block out]