the royal cavalry standard carried before the Roman emperors in war, esp. that first carried by Constantine, the first emperor to adopt Christianity: it usually bore the first two letters (XP) of the Greek Christos (Christ)
See labarum in American Heritage Dictionary 4
(lăbˈər-əm)
nounpl.lab·a·ra(-ər-ə)
An ecclesiastical banner, especially one carried in processions.
The banner adopted by Constantine I after his conversion to Christianity.