from
The Collaborative International Dictionary of English v.0.48
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.
[1913 Webster]