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]