The word “Xmas,” is used as an abbreviation of the word “Christmas.” The letter “X” comes from the Greek letter “chi,” the first letter in the Greek word for “Christ,” and “maesse” from the Old English word for “mass.” The word is considered as an informal form of the word “Christmas” and, as such, should not be used in formal writing. It has been in use since the year 1100 written as “Xpes maesse.” It evolved into the word “Xmas” during the middle of the 18th century.