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]