*labarum*

‖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 (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. [ 1913 Webster ]