labarum

n. ecclesiastical banner carried in processions; banner adopted by Constantine I after he converted to Christianity

The labarum was a military standard which displayed the first two Greek letters of the word Christ ( Greek: ΧΡΙΣΤΟΣ or Χριστός )—Chi (χ) and Rho, (ρ). It was first used by the Roman emperor Constantine I (Greek: Μέγας Κωνσταντίνος ).The etymology of the word before Constantine's usage of it is unclear.
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n. labarum

(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 (CHR) of the name of Christ in its Greek form. Later, the name was given to various modifications of this standard.
  

Labarum (Latin) Labaron (Greek) The standard carried by armies under the Christian Roman emperors, consisting of a pike from whose crossbar hung a silken banner bearing a crown of gold which enclosed the monogram made of the two first Greek letters of Christos. The banner was a modification of the original Roman banner, and the monogram was an ancient symbol used in the Mysteries, for instance in Egypt, where it was an emblem of Osiris and Horus ages before the Christian emperors. It is one form of the cross and circle, borrowed by the Christians and adapted to suit their purposes.
This symbol can be traced "from our modern cathedrals down to the Temple of Solomon, to the Egyptian Karnac, 1600 BC. The Thebans find it in the oldest Coptic records of symbols preserved on tablets of stone and recognize it, varying its multitudinous forms with every epoch, every people, creed or worship. It is a Rosicrucian symbol, one of the most ancient and the most mysterious. As the Egyptian Crux ansata,
or that travelled from India, where it was considered as belonging to the Indian symbolism of the most early ages, its lines and curves could be suited to answer the purpose of many symbols in every age and fitted for every worship" (Some Unpublished Letters of Blavatsky 153-5).