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. { X rays, or X´-rays• }, n. pl. The Röntgen rays; — so called by their discoverer because of their enigmatical character.