LABARUM
Etymology
Noun
labarum (plural labara)
The Roman military standard adopted by Constantine I. The banner was known for its Christian chi-rho sign - ☧.
Source: Wiktionary
Lab"a*rum (, n.; pl. Labara. Etym: [L.]
Definition: 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.
Source: Webster’s Unabridged Dictionary 1913 Edition