Definitions for XMASˈkrɪs məs; often ˈɛks məs
Random House Webster's College Dictionary
Xmas*ˈkrɪs məs; often ˈɛks məs
* Usage: The abbreviation Xmas for Christmas dates from the mid-16th cent. The X is the Greek letter chi, the initial in the word Χρ॓στός (Christos) “Christ.” In spite of a long and respectable history, today Xmas is objectionable to many, perhaps because of its associations with advertising. It is not used in formal writing.
Christmas, Christmas Day, Xmas, Dec 25(noun)
a Christian holiday celebrating the birth of Christ; a quarter day in England, Wales, and Ireland
Kernerman English Learner's Dictionary