The Collaborative International Dictionary of English v.0.48:

Christmas \Christ"mas\, n. [Christ + mass.]
   An annual church festival (December 25) and in some States a
   legal holiday, in memory of the birth of Christ, often
   celebrated by a particular church service, and also by
   special gifts, greetings, and hospitality.
   [1913 Webster]

   Christmas box.
   (a) A box in which presents are deposited at Christmas.
   (b) A present or small gratuity given to young people and
       servants at Christmas; a Christmas gift.

   Christmas carol, a carol sung at, or suitable for,
      Christmas.

   Christmas day. Same as Christmas.

   Christmas eve, the evening before Christmas.

   Christmas fern (Bot.), an evergreen North American fern
      (Aspidium acrostichoides), which is much used for
      decoration in winter.

   Christmas flower, Christmas rose, the black hellebore, a
      poisonous plant of the buttercup family, which in Southern
      Europe often produces beautiful roselike flowers
      midwinter.

   Christmas tree, a small evergreen tree, set up indoors, to
      be decorated with bonbons, presents, etc., and illuminated
      on Christmas eve.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Christmas Day
    n 1: a Christian holiday celebrating the birth of Christ; a
         quarter day in England, Wales, and Ireland [syn:
         Christmas, Christmas Day, Xmas, Dec 25]