Where does the X in "Xmas" come from?

The X in “Xmas” comes from the Greek letter Chi, the first letter of Christos, the Greek word for “Christ.”

Early Christians used Χ as a written abbreviation for Christ, much as scribes shortened other frequently used religious words. The abbreviation appeared in Christian manuscripts, artwork, and symbols, including the Chi-Rho monogram, which combines Chi (Χ) and Rho (Ρ), the first two letters of Christos.

The “-mas” part comes from “Mass,” referring to the Christian worship service celebrating Christ’s birth. Thus, “Xmas” is an abbreviation of “Christmas,” not a replacement that removes the holiday’s religious meaning. The practice has centuries of Christian usage behind it.

A common mix-up is confusing Chi with the Greek letter Xi. Chi is written Χ in uppercase and resembles the Latin letter X; Xi is written Ξ. Another misconception is that Xmas is a modern commercial invention. English abbreviations using X or Xp for “Christ” were documented by at least the late 15th century, although “Xmas” became especially familiar much later.