In the context of Christmas, "X" is often used as a shorthand for the phrase "Christ's Cross" or "the Cross of Christ." It symbolizes the sacrifice that Jesus Christ made for the salvation of humanity, which is a central theme of Christmas. The X represents the cross that Jesus was crucified on, and it serves as a reminder of the importance of his death and resurrection in the Christian faith. The X is also used as a shorthand for the Greek word "chi," which is the first letter of the Greek word for Christ.