Chi-Rho (Christogram, Labarum) Symbol
The Chi-Rho (pronounced "KEE-roe") is a Christian symbol consisting of the intersection of the capital Greek letters Chi (Χ) and Rho (Ρ), which are the first two letters of "Christ" in Greek (ΧΡΙΣΤΟΣ, Christos). The Chi-Rho can represent either Christ or Christianity and is also known as a Christogram.
The Chi-Rho is also known as the labarum, which is based on the Latin word laureum, "military standard." This is based on Constantine's application of the Chi-Rho symbol to his military standard after receiving his famous vision before the Battle of the Milvian Bridge in 312.
The church historian Eusebius, who spoke with the emperor about his experience, described the standard:
"On the top of the whole was fixed a wreath of gold and precious stones, and within this the symbol of the Saviour's name, two letters indicating the name of Christ by means of the initial letters, the letter X intersection P at the center."
The Chi-Rho was used by Christians before Constantine but it became much more widespread after he adopted it. The chi-rho appeared on the coins of Constantine and his Christian successors, sometimes alone and sometimes as part of a military standard as described by Eusebius. It continues to be widely used today.