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.
      [1913 Webster]

   2. To disguise; to cover; to hide.
      [1913 Webster]

            Masking the business from the common eye. --Shak.
      [1913 Webster]

   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]

WordNet (r) 3.0 (2006):

masking
    n 1: the act of concealing the existence of something by
         obstructing the view of it; "the cover concealed their guns
         from enemy aircraft" [syn: cover, covering,
         screening, masking]
    2: the blocking of one sensation resulting from the presence of
       another sensation; "he studied auditory masking by pure
       tones"
    3: scenery used to block the audience's view of parts of the
       stage that should not be seen [syn: masking piece,
       masking]