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.
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   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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WordNet (r) 3.0 (2006):

death mask
    n 1: a cast taken from the face of a dead person