What is "constantinian symbol"

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.