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.

Source: Webster's Revised Unabridged Dictionary (1913)