lab·a·rum (l?b??r-?m)n. pl. lab·a·ra (-?r-?) 1. An ecclesiastical banner, especially one carried in processions.2. The banner adopted by Constantine I after his conversion to Christianity.[Late Latin, probably from alteration of Greek labr?ton, laurel-leaf standard, from Latin laure?tum, neuter of laure?tus, adorned with laurel; see laureate.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.labarum (?læb?r?m) n, pl -ra (-r?) 1. (Ecclesiastical Terms) a standard or banner carried in Christian religious processions2. (Historical Terms) the military standard bearing a Christian monogram used by Constantine the Great3. (Military) the military standard bearing a Christian monogram used by Constantine the Great[C17: from Late Latin, of obscure origin]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014lab?a?rum (?læb ?r ?m) n., pl. -a?ra (-?r ?) the military standard of Constantine the Great and later Christian emperors of Rome, bearing Christian symbols. [1650?60;