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.

An acronym for eXtreme Programming.  It defines a process for agile software development that is very OOP-focused.