labarum

noun

1. The Roman military standard adopted by Constantine I. The banner was known for its Christian chi-rho sign - ☧.

   “1658: Nor shall we urge the labarum, and famous Standard of Constantine, or make further use thereof, than as the first Letters in the Name of our Saviour Christ — Sir Thomas Browne, The Garden of Cyrus (Folio Society 2007, p. 168)”