The Collaborative International Dictionary of English v.0.48:

Xmas \X"mas\, n.
   Christmas; -- an abbreviation for the Christian holiday
   celebrating the birth of Christ; a quarter day in England,
   Wales, and Ireland.

   Syn: Christmas, Christmas Day, Dec 25.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Xmas
    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]