Webster's Revised Unabridged Dictionary · 1913

The first two letters of the Greek word CHRISTOS, 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.