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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.]