This chi rho (pronounced like
chiro as in chiropractor) appears
with the Alpha and Omega,
the first and last letters of the
I am the Alpha and the Omega,
the First and the Last,
the Beginning and the End. (NIV)
The Chi-Rho symbol is generally depicted as the Greek letter chi (which looks like the Roman alphabet X) superimposed over the elongated stem of the Greek letter rho (which looks like the Roman alphabet P). It is a Christian symbol representing Jesus Christ, with the chi and rho being the first two letters in the Greek spelling of Christ (Christos; chi-rho-iota-sigma-tau-omicron-sigma).
The Chi-Rho symbol is also called the labarum, the Monogram of Christ, Constantine's Cross, the Chrismon, the Christogram, and the Monogram of Constantine. The Chi-Rho symbol was used to represent Jesus even before 312 AD. However, in 312 AD, the Roman Emperor Constantine 1 was said to have had a vision or a dream of the Chi-Rho symbol in the sky in which were emblazoned the Greek words 'en touto Nika' (translated in Latin into 'In Hoc Signo Vinces' and in English into "In this sign (the cross) you will conquer."
Some believe that the arrangement of stars in the night sky of 27 October 312 formed the Chi-Rho. In any event, Constantine 1 had his troops' shields emblazoned with this symbol and created his own standard with the Chi-Rho and the 'en touto Nika' as prominent symbols. Constantine 1, although out manned, defeated his enemy and became sole emperor of the Roman Empire.