The Labarum (Greek: λάβαρον / láboron) was a Christian imperial standard incorporating the sacred "Chi-Rho" Christogram, which was one of the earliest forms of christogram used by Christians, becoming one of the most familiar and widely used emblems in Christian tradition. It was adapted by emperor Saint Constantine the Great after receiving his celestial vision and dream, on the eve of his victory at the Milvian Bridge in 313 AD.
The Labarum of Constantine was a vexillum that displayed the "Chi-Rho" Christogram, formed from the first two Greek letters of the word "Christ" (Greek: ΧΡΙΣΤΟΣ, or Χριστός) — Chi (χ) and Rho (ρ). Fashioned after legionary standards, it substituted the form of a cross for the old pagan symbols, and was surmounted by a jewelled wreath of gold containing the monogram of Christ, intersecting Chi (χ) and Rho (ρ); upon this hung a rich purple banner, beset with gold trim and profuse embroidery. The inscription "Εν Τουτω Νικα" (In Hoc Signo Vinces) — "In this sign, conquer" was in all probability inscribed upon the actual standard, although Eusebius mentions that royal portraits of Constantine and his children were integrated. St. Ambrose of Milan later wrote that the Labarum was consecrated by the Name of Christ.
As a new focal point for Roman unity, the monogram appeared on coins, shields, and later public buildings and churches. From 324 the Labarum with the "Chi-Rho" Christogram was the official standard of the Roman Empire.