What is "labarum"

Labarum \Lab"a*rum\, n.; pl. Labara. [L.] The standard adopted by the Emperor Constantine after his conversion to Christianity. It is described as a pike bearing a silk banner hanging from a crosspiece, and surmounted by a golden crown. It bore a monogram of the first two letters ([Chi][RHO]) (which appear like the English letters X and P), of the name of Christ in its Greek form. Later, the name was given to various modifications of this standard.

n. The Roman military standard adopted by http://en.wikipedia.org/wiki/Constantine%20I. The banner was known for its Christian chi-rho sign - (l mul ☧).

The labarum was a vexillum (military standard) that displayed the " Chi-Rho" symbol ☧, a christogram formed from the first two Greek letters of the word " Christ" (, or Χριστός) — Chi (χ) and Rho (ρ). It was first used by the Roman emperor Constantine I. Since the vexillum consisted of a flag suspended from the crossbar of a cross, it was ideally suited to symbolize the crucifixion of Christ.

Ancient sources draw an unambiguous distinction between the two terms "labarum" and "Chi-Rho", even though later usage sometimes regards the two as synonyms. The name labarum was applied both to the original standard used by Constantine the Great and to the many standards produced in imitation of it in the Late Antique world, and subsequently.