Xmas

Xmas is a common abbreviation of the word Christmas. It is sometimes pronounced /ˈɛksməs/, but Xmas, and variants such as Xtemass, originated as handwriting abbreviations for the typical pronunciation /ˈkrɪsməs/. The "X" comes from the Greek letter Chi, which is the first letter of the Greek word Χριστός, which in English is "Christ". The "-mas" part is from the Latin-derived Old English word for Mass.

There is a common belief that the word Xmas stems from a secular attempt to remove the religious tradition from Christmas by taking the "Christ" out of "Christmas", but its use dates back to the 16th century.

"Xmas" is deprecated by some modern style guides, including those at the New York Times,The Times, The Guardian, and the BBC.Millicent Fenwick, in the 1948 Vogue's Book of Etiquette, states that "'Xmas' should never be used" in greeting cards.The Cambridge Guide to Australian English Usage states that the spelling should be considered informal and restricted to contexts where concision is valued, such as headlines and greeting cards.The Christian Writer's Manual of Style, while acknowledging the ancient and respectful use of "Xmas" in the past, states that the spelling should never be used in formal writing.

...
Wikipedia