n. The Roman military standard adopted by Constantine I. The banner was known for its Christian chi-rho sign - ☧.
the GNU version of the Collaborative International Dictionary of English
n. 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 (ΧΡ) (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.
The Century Dictionary and Cyclopedia
n. A Roman military standard adopted by the later emperors as the imperial standard.
n. A standard or banner of similar form, borne in ecclesiastical processions of the Roman Catholic Church.
n. Figuratively, a moral standard, guide, or device.
Word Usage
"The term labarum, which is of uncertain derivation, was probably familiar in the Roman army from the reign of Hadrian."