A monogram of the name Christ.
From the third century the names of our Saviour are sometimes shortened, particularly in Christian inscriptions (IH and XP, for Jesus and Christus). In the next century the "sigla" (chi-rho) occurs not only as an abbreviation but also as a symbol.
It is the first two alphabets of the Greek name for Christ, 
It is the celebrated sign which appeared in the sky at noon-day to the
Emperor Constantine and
his troops, and was afterwards adopted by him on his standard; the
monogram is hence
sometimes called the Labarum, as well as the Cross of Constantine.