The Collaborative International Dictionary of English v.0.48:

Domino \Dom"i*no\, n.; pl. Dominos or (esp. the pieces for a
   game) Dominoes. [F. domino, or It. domin[`o], or Sp.
   domin['o], fr. L. dominus master. The domino was orig. a hood
   worn by the canons of a cathedral. See Don, Dame.]
   1. A kind of hood worn by the canons of a cathedral church; a
      sort of amice. --Kersey.
      [1913 Webster]

   2. A mourning veil formerly worn by women.
      [1913 Webster]

   3. A kind of mask; particularly, a half mask worn at
      masquerades, to conceal the upper part of the face.
      Dominos were formerly worn by ladies in traveling.
      [1913 Webster]

   4. A costume worn as a disguise at masquerades, consisting of
      a robe with a hood adjustable at pleasure.
      [1913 Webster]

   5. A person wearing a domino.
      [1913 Webster]

   6. pl. A game played by two or more persons, with
      twenty-eight pieces of wood, bone, or ivory, of a flat,
      oblong shape, plain at the back, but on the face divided
      by a line in the middle, and either left blank or
      variously dotted after the manner of dice. The game is
      played by matching the spots or the blank of an unmatched
      half of a domino already played --Hoyle.
      [1913 Webster]

   7. One of the pieces with which the game of dominoes is
      played. --Hoyle.

   fall like dominoes. To fall sequentially, as when one
      object in a line, by falling against the next object,
      causes it in turn to fall, and that second object causes a
      third to fall, etc.; the process can be repeated an
      indefinite number of times.

   Note: The phrase is derived from an entertainment using
         dominoes arranged in a row, each standing on edge and
         therefore easily knocked over; when the first is made
         to fall against the next, it starts a sequence which
         ends when all have fallen. For amusement, people have
         arranged such sequences involving thousands of
         dominoes, arrayed in fanciful patterns.

WordNet (r) 3.0 (2006):

dominos
    n 1: any of several games played with small rectangular blocks
         [syn: dominoes, dominos]