labarum

An ecclesiastical banner, especially one carried in processions.

The banner adopted by Constantine I after his conversion to Christianity.

A Roman military standard adopted by the later emperors as the imperial standard.

A standard or banner of similar form, borne in ecclesiastical processions of the Roman Catholic Church.

Figuratively, a moral standard, guide, or device.

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 Roman military standard adopted by Constantine I. The banner was known for its Christian chi-rho sign – ☧.