Part of the Chi-roh (Greek symbol, I think)
which stand for Christ.
Its two greek letters together, Chi and Rho (X and R) that to this day translate into Christ in Latin and Greek rite Catholic churches. Why the X (Chi) was ever substituted alone, to represent the entire name of Christ by itself, I don't know.
To us its Christ and Christmas.
Hope everyone is having a blessed Advent season.
This is link to Christian symbols. Chi Rho is the 4th down on the right side. http://home.rochester.rr.com/gocek/images/christn/
In the early 4th century AD Christianity was adopted as the official religion of the Roman empire. In Britain Christian symbols have been found as evidence of Christianity. The main motifs are the Greek letters X and P, known as the 'chi-rho' symbol, which begin the name of Christ. (Source)