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.]