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.
      [1913 Webster]

   2. That which disguises; a pretext or subterfuge.
      [1913 Webster]

   3. A festive entertainment of dancing or other diversions,
      where all wear masks; a masquerade; hence, a revel; a
      frolic; a delusive show. --Bacon.
      [1913 Webster]

            This thought might lead me through the world's vain
            mask.                                 --Milton.
      [1913 Webster]

   4. A dramatic performance, formerly in vogue, in which the
      actors wore masks and represented mythical or allegorical
      characters.
      [1913 Webster]

   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.
      [1913 Webster]

   6. (Fort.)
      (a) In a permanent fortification, a redoubt which protects
          the caponiere.
      (b) A screen for a battery.
          [1913 Webster]

   7. (Zool.) The lower lip of the larva of a dragon fly,
      modified so as to form a prehensile organ.
      [1913 Webster]

   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.
      [1913 Webster]

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.
      [1913 Webster]

            They must all be masked and vizarded. --Shak.
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   2. To disguise; to cover; to hide.
      [1913 Webster]

            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.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mask \Mask\, v. i.
   1. To take part as a masker in a masquerade. --Cavendish.
      [1913 Webster]

   2. To wear a mask; to be disguised in any way. --Shak.
      [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

280 Moby Thesaurus words for "mask":
   Kaffeeklatsch, aegis, affectation, air, alibi, anaglyph, apology,
   appearance, apply to, arm guard, aspect, backstop, bal,
   bal costume, bal masque, ball, barn dance, bas-relief, becloud,
   befog, belie, blanket, blind, block, blowout, blur, boss, buffer,
   bulwark, bumper, cameo, cameo glass, camouflage, canopy,
   cavo-rilievo, cloak, clothe, cloud, coat, cocktail party,
   coffee klatch, color, coloring, conceal, concealment,
   contraceptive, cope, copyright, costume party, country dance,
   cover, cover story, cover up, cover-up, coverage, covering, covert,
   coverture, cowl, cowling, crash helmet, curtain, cushion,
   cut glass, dance, dashboard, defend, device, dinner, dinner party,
   disguise, dissemble, dissembling, dissimulate, dissimulation,
   distort, distract attention from, dodger, domino, donation party,
   drape, drapery, dress up, eclipse, embellish, embossment,
   embroider, ensconce, enshroud, entertainment, envelop, excuse,
   facade, face, face mask, fakery, false colors, false face,
   false front, falsify, fancy-dress ball, feint, fender, festivity,
   film, finger guard, foot guard, front, fudge, fuse, garble,
   garden party, gild, gloss, gloss over, glyph, glyptograph, goggles,
   governor, guard, guardrail, guise, hand guard, handle, handrail,
   hanging, hard hat, helmet, hen party, hide, high relief, hood, hop,
   house party, house-raising, housewarming, housing, incognito,
   insulation, intaglio, intaglio rilevato, intaglio rilievo,
   interlock, keep under cover, knee guard, knuckle guard,
   lame excuse, laminated glass, lawn party, lay on, lay over,
   life preserver, lifeline, lightning conductor, lightning rod,
   locus standi, low relief, mantle, masked ball, masque, masquerade,
   masquerade ball, masquerade party, medal, medallion, miscite,
   miscolor, misquote, misreport, misrepresent, misstate, mixer,
   mudguard, muffle, muffler, mummery, nose guard, obduce, obfuscate,
   obscure, occult, ostensible motive, overlay, overspread, overstate,
   pad, padding, pall, palladium, party, patent, pervert, pilot,
   plaquette, poor excuse, pose, posture, pretense, pretension,
   pretext, preventive, prom, promenade, prophylactic, protect,
   protective clothing, protective coloration, protective umbrella,
   protestation, public motive, put on, put-off, put-on, record hop,
   refuge, relief, relievo, safeguard, safety, safety glass,
   safety plug, safety rail, safety shoes, safety switch,
   safety valve, screen, sculptured glass, scum, seat belt, secrete,
   seeming, semblance, shade, sham, shelter, shield, shin guard,
   shindig, shindy, show, shower, shroud, simulation, slant,
   slur over, smoke screen, smoker, spread over, square dance, stag,
   stag dance, stag party, stalking-horse, strain, stratagem,
   subterfuge, sun helmet, superimpose, superpose, surprise party,
   tea dance, the dansant, titivate, trick, trick out, twist,
   umbrella, understate, varnish, veil, veneer, vestment, visor,
   vizard, vizard mask, warp, whitewash, window dressing, windscreen,
   windshield