Constantinian symbol

from The Collaborative International Dictionary of English v.0.48

XP \XP\ [Belongs here in appearance only.]
   The first two letters of the Greek word XRISTOS, Christ; --
   an abbreviation used with the letters separate or, oftener,
   in a monogram, often inclosed in a circle, as a symbol or
   emblem of Christ. It use as an emblem was introduced by
   Constantine the Great, whence it is known as the

   {Constantinian symbol}, or

   {monogram}. See {Labarum}.
      [Webster 1913 Suppl.]