The Collaborative International Dictionary of English v.0.48:

Masked \Masked\, a.
   1. Wearing a mask or masks; characterized by masks;
      concealed; hidden.
      [1913 Webster]

   2. (Bot.) Same as Personate.
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   3. (Zool.) Having the anterior part of the head differing
      decidedly in color from the rest of the plumage; -- said
      of birds.
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   Masked ball, a ball in which the dancers wear masks.

   Masked battery (Mil.), a battery so placed as not to be
      seen by an enemy until it opens fire. --H. L. Scott.

   Masked crab (Zool.), a European crab (Corystes
      cassivelaunus) with markings on the carapace somewhat
      resembling a human face.

   Masked pig (Zool.), a Japanese domestic hog (Sus
      pliciceps). Its face is deeply furrowed.
      [1913 Webster]

WordNet (r) 3.0 (2006):

masked ball
    n 1: a ball at which guests wear costumes and masks [syn:
         masked ball, masquerade ball, fancy-dress ball]